Support 256-color terminal environments

Fixes #110

With this commit, delta enters "24 bit color mode" iff either of the
following are true:

1. The --24-bit-color command line option value is "always"

2. The --24-bit-color command line option value is "auto" and the
   COLORTERM environment variable is set to "truecolor" or "24bit".
   See
   https://gist.github.com/XVilka/8346728#true-color-detection
   https://github.com/sharkdp/bat/blob/7779d9f6221b3e98c43a43ceb5596ba285fdf4f8/src/bin/bat/app.rs#L29-L33

Otherwise, delta enters "8-bit color mode".

In "24 bit color mode", delta will

1. Emit 24-bit RGB color shell escape sequences that will only be
   displayed correctly by a terminal application that supports 24 bit
   colors.

2. Select default background colors that will only be displayed
   correctly by a terminal application that supports 24 bit colors.

In "8-bit color mode", delta will

1. Emit color shell escape sequences specifying the entry in the ANSI
   256 color palette that is closest (according to the ansi_colours
   library) to the requested color.

2. Select default background colors that will work well in a terminal
   application that supports 8-bit color but not 24-bit color.

9 files changed, 241 insertions, 85 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 8654d5ae..cf2c6348 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -14,6 +14,14 @@ dependencies = [
 ]
 
 [[package]]
+name = "ansi_colours"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "cc 1.0.38 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "ansi_term"
 version = "0.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -273,6 +281,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 name = "git-delta"
 version = "0.0.16"
 dependencies = [
+ "ansi_colours 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "atty 0.2.13 (registry+https://github.com/rust-lang/crates.io-index)",
  "box_drawing 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -829,6 +838,7 @@ dependencies = [
 [metadata]
 "checksum adler32 1.0.3 (registry+https://github.com/rust-lang/crates.io-index)" = "7e522997b529f05601e05166c07ed17789691f562762c7f3b987263d2dedee5c"
 "checksum aho-corasick 0.7.6 (registry+https://github.com/rust-lang/crates.io-index)" = "58fb5e95d83b38284460a5fda7d6470aa0b8844d283a0b614b8535e880800d2d"
+"checksum ansi_colours 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "1d0f302a81afc6a7f4350c04f0ba7cfab529cc009bca3324b3fb5764e6add8b6"
 "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
 "checksum arrayref 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)" = "0d382e583f07208808f6b1249e60848879ba3543f57c32277bf52d69c2f0f0ee"
 "checksum arrayvec 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cff77d8686867eceff3105329d4698d96c2391c176d5d03adc90c7389162b5b8"
diff --git a/Cargo.toml b/Cargo.toml
index fb7d071a..25ff717b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,6 +16,7 @@ name = "delta"
 path = "src/main.rs"
 
 [dependencies]
+ansi_colours = "1.0.1"
 ansi_term = "0.11.0"
 atty = "0.2.13"
 box_drawing = "0.1.2"
diff --git a/src/cli.rs b/src/cli.rs
index f896c492..3337f774 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -8,6 +8,7 @@ use structopt::StructOpt;
 use crate::bat::assets::HighlightingAssets;
 use crate::bat::output::PagingMode;
 use crate::config;
+use crate::env;
 use crate::style;
 
 #[derive(StructOpt, Clone, Debug)]
@@ -155,6 +156,14 @@ pub struct Opt {
     #[structopt(long = "max-line-distance", default_value = "0.3")]
     pub max_line_distance: f64,
 
+    /// Whether to emit 24-bit ("true color") RGB color codes. Options are auto, always, and never.
+    /// "auto" means that delta will emit 24-bit color codes iff the environment variable COLORTERM
+    /// has the value "truecolor" or "24bit". If your terminal application (the application you use
+    /// to enter commands at a shell prompt) supports 24 bit colors, then it probably already sets
+    /// this environment variable, in which case you don't need to do anything.
+    #[structopt(long = "24-bit-color", default_value = "auto")]
+    pub true_color: String,
+
     /// Whether to use a pager when displaying output. Options are: auto, always, and never. The
     /// default pager is `less`: this can be altered by setting the environment variables BAT_PAGER
     /// or PAGER (BAT_PAGER has priority).
@@ -257,12 +266,32 @@ pub fn process_command_line_arguments<'a>(
         }
     };
 
+    let true_color = match opt.true_color.as_ref() {
+        "always" => true,
+        "never" => false,
+        "auto" => is_truecolor_terminal(),
+        _ => {
+            eprintln!(
+                "Invalid value for --24-bit-color option: {} (valid values are \"always\", \"never\", and \"auto\")",
+                opt.true_color
+            );
+            process::exit(1);
+        }
+    };
+
     config::get_config(
         opt,
         &assets.syntax_set,
         &assets.theme_set,
+        true_color,
         available_terminal_width,
         background_color_width,
         paging_mode,
     )
 }
+
+fn is_truecolor_terminal() -> bool {
+    env::get_env_var("COLORTERM")
+        .map(|colorterm| colorterm == "truecolor" || colorterm == "24bit")
+        .unwrap_or(false)
+}
diff --git a/src/config.rs b/src/config.rs
index 3c3f033b..2a6590aa 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -4,11 +4,11 @@ use std::str::FromStr;
 use syntect::highlighting::{Color, Style, StyleModifier, Theme, ThemeSet};
 use syntect::parsing::SyntaxSet;
 
+use crate::bat::output::PagingMode;
 use crate::cli;
 use crate::env;
 use crate::paint;
 use crate::style;
-use crate::bat::output::{PagingMode};
 
 pub struct Config<'a> {
     pub theme: Option<&'a Theme>,
@@ -22,6 +22,7 @@ pub struct Config<'a> {
     pub hunk_color: Color,
     pub syntax_set: &'a SyntaxSet,
     pub terminal_width: usize,
+    pub true_color: bool,
     pub width: Option<usize>,
     pub tab_width: usize,
     pub opt: &'a cli::Opt,
@@ -34,6 +35,7 @@ pub fn get_config<'a>(
     opt: &'a cli::Opt,
     syntax_set: &'a SyntaxSet,
     theme_set: &'a ThemeSet,
+    true_color: bool,
     terminal_width: usize,
     width: Option<usize>,
     paging_mode: PagingMode,
@@ -53,11 +55,9 @@ pub fn get_config<'a>(
     };
 
     let minus_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.minus_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_MINUS_COLOR,
-            style::DARK_THEME_MINUS_COLOR,
+            style::get_minus_color_default(is_light_mode, true_color),
         )),
         foreground: if opt.highlight_removed {
             None
@@ -68,11 +68,9 @@ pub fn get_config<'a>(
     };
 
     let minus_emph_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.minus_emph_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_MINUS_EMPH_COLOR,
-            style::DARK_THEME_MINUS_EMPH_COLOR,
+            style::get_minus_emph_color_default(is_light_mode, true_color),
         )),
         foreground: if opt.highlight_removed {
             None
@@ -83,22 +81,18 @@ pub fn get_config<'a>(
     };
 
     let plus_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.plus_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_PLUS_COLOR,
-            style::DARK_THEME_PLUS_COLOR,
+            style::get_plus_color_default(is_light_mode, true_color),
         )),
         foreground: None,
         font_style: None,
     };
 
     let plus_emph_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.plus_emph_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_PLUS_EMPH_COLOR,
-            style::DARK_THEME_PLUS_EMPH_COLOR,
+            style::get_plus_emph_color_default(is_light_mode, true_color),
         )),
         foreground: None,
         font_style: None,
@@ -114,6 +108,7 @@ pub fn get_config<'a>(
         commit_color: color_from_rgb_or_ansi_code(&opt.commit_color),
         file_color: color_from_rgb_or_ansi_code(&opt.file_color),
         hunk_color: color_from_rgb_or_ansi_code(&opt.hunk_color),
+        true_color,
         terminal_width,
         width,
         tab_width: opt.tab_width,
@@ -192,20 +187,9 @@ fn color_from_rgb_or_ansi_code(s: &str) -> Color {
     }
 }
 
-fn color_from_arg_or_mode_default(
-    arg: Option<&String>,
-    is_light_mode: bool,
-    light_theme_default: Color,
-    dark_theme_default: Color,
-) -> Color {
+fn color_from_arg(arg: Option<&String>, default: Color) -> Color {
     match arg {
         Some(string) => color_from_rgb_or_ansi_code(&string),
-        None => {
-            if is_light_mode {
-                light_theme_default
-            } else {
-                dark_theme_default
-            }
-        }
+        None => default,
     }
 }
diff --git a/src/delta.rs b/src/delta.rs
index e68b8bd0..8d36db58 100644
--- a/src/delta.rs
+++ b/src/delta.rs
@@ -204,6 +204,7 @@ fn handle_commit_meta_header_line(
         config.terminal_width,
         config.commit_color,
         true,
+        config.true_color,
     )?;
     Ok(())
 }
@@ -234,10 +235,11 @@ fn handle_generic_file_meta_header_line(
     writeln!(painter.writer)?;
     draw_fn(
         painter.writer,
-        &paint::paint_text_foreground(line, config.file_color),
+        &paint::paint_text_foreground(line, config.file_color, config.true_color),
         config.terminal_width,
         config.file_color,
         false,
+        config.true_color,
     )?;
     Ok(())
 }
@@ -279,13 +281,14 @@ fn handle_hunk_meta_line(
             config.terminal_width,
             config.hunk_color,
             false,
+            config.true_color,
         )?;
         painter.output_buffer.clear();
     }
     writeln!(
         painter.writer,
         "\n{}",
-        paint::paint_text_foreground(line_number, config.hunk_color)
+        paint::paint_text_foreground(line_number, config.hunk_color, config.true_color)
     )?;
     Ok(())
 }
@@ -431,6 +434,7 @@ mod tests {
 
     #[test]
     fn test_theme_selection() {
+        #[derive(PartialEq)]
         enum Mode {
             Light,
             Dark,
@@ -496,6 +500,7 @@ mod tests {
             } else {
                 env::set_var("BAT_THEME", bat_theme_env_var);
             }
+            let is_true_color = true;
             let mut options = get_command_line_options();
             options.theme = theme_option;
             match mode_option {
@@ -521,31 +526,19 @@ mod tests {
             }
             assert_eq!(
                 config.minus_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_MINUS_COLOR,
-                    Mode::Dark => style::DARK_THEME_MINUS_COLOR,
-                }
+                style::get_minus_color_default(expected_mode == Mode::Light, is_true_color)
             );
             assert_eq!(
                 config.minus_emph_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_MINUS_EMPH_COLOR,
-                    Mode::Dark => style::DARK_THEME_MINUS_EMPH_COLOR,
-                }
+                style::get_minus_emph_color_default(expected_mode == Mode::Light, is_true_color)
             );
             assert_eq!(
                 config.plus_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_PLUS_COLOR,
-                    Mode::Dark => style::DARK_THEME_PLUS_COLOR,
-                }
+                style::get_plus_color_default(expected_mode == Mode::Light, is_true_color)
             );
             assert_eq!(
                 config.plus_emph_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_PLUS_EMPH_COLOR,
-                    Mode::Dark => style::DARK_THEME_PLUS_EMPH_COLOR,
-                }
+                style::get_plus_emph_color_default(expected_mode == Mode::Light, is_true_color)
             );
         }
     }
@@ -580,7 +573,7 @@ mod tests {
     fn paint_text(input: &str, style_modifier: StyleModifier, config: &Config) -> String {
         let mut output = String::new();
         let style = config.no_style.apply(style_modifier);
-        paint::paint_text(&input, style, &mut output);
+        paint::paint_text(&input, style, &mut output, config.true_color);
         output
     }
 
@@ -623,6 +616,7 @@ mod tests {
             file_color: "Blue".to_string(),
             hunk_style: cli::SectionStyle::Box,
             hunk_color: "blue".to_string(),
+            true_color: "always".to_string(),
             width: Some("variable".to_string()),
             paging_mode: "auto".to_string(),
             tab_width: 4,
diff --git a/src/draw.rs b/src/draw.rs
index 751f99f9..03b93bdb 100644
--- a/src/draw.rs
+++ b/src/draw.rs
@@ -15,6 +15,7 @@ pub fn write_boxed(
     _line_width: usize, // ignored
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let up_left = if heavy {
         box_drawing::heavy::UP_LEFT
@@ -22,8 +23,12 @@ pub fn write_boxed(
         box_drawing::light::UP_LEFT
     };
     let box_width = strip_ansi_codes(text).graphemes(true).count() + 1;
-    write_boxed_partial(writer, text, box_width, color, heavy)?;
-    write!(writer, "{}", paint::paint_text_foreground(up_left, color))?;
+    write_boxed_partial(writer, text, box_width, color, heavy, true_color)?;
+    write!(
+        writer,
+        "{}",
+        paint::paint_text_foreground(up_left, color, true_color)
+    )?;
     Ok(())
 }
 
@@ -35,9 +40,10 @@ pub fn write_boxed_with_line(
     line_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let box_width = strip_ansi_codes(text).graphemes(true).count() + 1;
-    write_boxed_with_horizontal_whisker(writer, text, box_width, color, heavy)?;
+    write_boxed_with_horizontal_whisker(writer, text, box_width, color, heavy, true_color)?;
     write_horizontal_line(
         writer,
         if line_width > box_width {
@@ -47,6 +53,7 @@ pub fn write_boxed_with_line(
         },
         color,
         heavy,
+        true_color,
     )?;
     write!(writer, "\n")?;
     Ok(())
@@ -58,9 +65,14 @@ pub fn write_underlined(
     line_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
-    writeln!(writer, "{}", paint::paint_text_foreground(text, color))?;
-    write_horizontal_line(writer, line_width - 1, color, heavy)?;
+    writeln!(
+        writer,
+        "{}",
+        paint::paint_text_foreground(text, color, true_color)
+    )?;
+    write_horizontal_line(writer, line_width - 1, color, heavy, true_color)?;
     write!(writer, "\n")?;
     Ok(())
 }
@@ -70,6 +82,7 @@ fn write_horizontal_line(
     line_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let horizontal = if heavy {
         box_drawing::heavy::HORIZONTAL
@@ -79,7 +92,7 @@ fn write_horizontal_line(
     write!(
         writer,
         "{}",
-        paint::paint_text_foreground(&horizontal.repeat(line_width), color)
+        paint::paint_text_foreground(&horizontal.repeat(line_width), color, true_color)
     )
 }
 
@@ -89,17 +102,18 @@ pub fn write_boxed_with_horizontal_whisker(
     box_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let up_horizontal = if heavy {
         box_drawing::heavy::UP_HORIZONTAL
     } else {
         box_drawing::light::UP_HORIZONTAL
     };
-    write_boxed_partial(writer, text, box_width, color, heavy)?;
+    write_boxed_partial(writer, text, box_width, color, heavy, true_color)?;
     write!(
         writer,
         "{}",
-        paint::paint_text_foreground(up_horizontal, color)
+        paint::paint_text_foreground(up_horizontal, color, true_color)
     )?;
     Ok(())
 }
@@ -110,6 +124,7 @@ fn write_boxed_partial(
     box_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let horizontal = if heavy {
         box_drawing::heavy::HORIZONTAL
@@ -131,10 +146,10 @@ fn write_boxed_partial(
     write!(
         writer,
         "{}{}\n{} {}\n{}",
-        paint::paint_text_foreground(&horizontal_edge, color),
-        paint::paint_text_foreground(down_left, color),
-        paint::paint_text_foreground(text, color),
-        paint::paint_text_foreground(vertical, color),
-        paint::paint_text_foreground(&horizontal_edge, color),
+        paint::paint_text_foreground(&horizontal_edge, color, true_color),
+        paint::paint_text_foreground(down_left, color, true_color),
+        paint::paint_text_foreground(text, color, true_color),
+        paint::paint_text_foreground(vertical, color, true_color),
+        paint::paint_text_foreground(&horizontal_edge, color, true_color),
     )
 }
diff --git a/src/main.rs b/src/main.rs
index fea30115..79f00ea5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -82,16 +82,26 @@ fn show_background_colors(config: &config::Config) {
          --minus-emph-color=\"{minus_emph_color}\" \
          --plus-color=\"{plus_color}\" \
          --plus-emph-color=\"{plus_emph_color}\"",
-        minus_color = get_painted_rgb_string(config.minus_style_modifier.background.unwrap()),
-        minus_emph_color =
-            get_painted_rgb_string(config.minus_emph_style_modifier.background.unwrap()),
-        plus_color = get_painted_rgb_string(config.plus_style_modifier.background.unwrap()),
-        plus_emph_color =
-            get_painted_rgb_string(config.plus_emph_style_modifier.background.unwrap()),
+        minus_color = get_painted_rgb_string(
+            config.minus_style_modifier.background.unwrap(),
+            config.true_color
+        ),
+        minus_emph_color = get_painted_rgb_string(
+            config.minus_emph_style_modifier.background.unwrap(),
+            config.true_color
+        ),
+        plus_color = get_painted_rgb_string(
+            config.plus_style_modifier.background.unwrap(),
+            config.true_color
+        ),
+        plus_emph_color = get_painted_rgb_string(
+            config.plus_emph_style_modifier.background.unwrap(),
+            config.true_color
+        ),
     )
 }
 
-fn get_painted_rgb_string(color: Color) -> String {
+fn get_painted_rgb_string(color: Color, true_color: bool) -> String {
     let mut string = String::new();
     let style = Style {
         foreground: style::NO_COLOR,
@@ -102,6 +112,7 @@ fn get_painted_rgb_string(color: Color) -> String {
         &format!("#{:02x?}{:02x?}{:02x?}", color.r, color.g, color.b),
         style,
         &mut string,
+        true_color,
     );
     string.push_str("\x1b[0m"); // reset
     string
diff --git a/src/paint.rs b/src/paint.rs
index cf6968e1..f85c64a1 100644
--- a/src/paint.rs
+++ b/src/paint.rs
@@ -1,6 +1,7 @@
 use std::io::Write;
 use std::str::FromStr;
 
+use ansi_colours;
 use syntect::easy::HighlightLines;
 use syntect::highlighting::{Color, Style, StyleModifier};
 use syntect::parsing::{SyntaxReference, SyntaxSet};
@@ -109,7 +110,7 @@ impl<'a> Painter<'a> {
         {
             let mut text_width = 0;
             for (style, text) in superimpose_style_sections(syntax_sections, diff_sections) {
-                paint_text(&text, style, output_buffer);
+                paint_text(&text, style, output_buffer, config.true_color);
                 if config.width.is_some() {
                     text_width += text.graphemes(true).count();
                 }
@@ -127,6 +128,7 @@ impl<'a> Painter<'a> {
                             &" ".repeat(width - text_width),
                             background_style,
                             output_buffer,
+                            config.true_color,
                         );
                     }
                     _ => (),
@@ -206,22 +208,34 @@ impl<'a> Painter<'a> {
 }
 
 /// Write section text to buffer with shell escape codes specifying foreground and background color.
-pub fn paint_text(text: &str, style: Style, output_buffer: &mut String) {
+pub fn paint_text(text: &str, style: Style, output_buffer: &mut String, true_color: bool) {
     if text.is_empty() {
         return;
     }
     if style.background != style::NO_COLOR {
-        output_buffer.push_str(&get_color_escape_sequence(style.background, false));
+        output_buffer.push_str(&get_color_escape_sequence(
+            style.background,
+            false,
+            true_color,
+        ));
     }
     if style.foreground != style::NO_COLOR {
-        output_buffer.push_str(&get_color_escape_sequence(style.foreground, true));
+        output_buffer.push_str(&get_color_escape_sequence(
+            style.foreground,
+            true,
+            true_color,
+        ));
     }
     output_buffer.push_str(text);
 }
 
 /// Return text together with shell escape codes specifying the foreground color.
-pub fn paint_text_foreground(text: &str, color: Color) -> String {
-    format!("{}{}", get_color_escape_sequence(color, true), text)
+pub fn paint_text_foreground(text: &str, color: Color, true_color: bool) -> String {
+    format!(
+        "{}{}",
+        get_color_escape_sequence(color, true, true_color),
+        text,
+    )
 }
 
 /// Return shell escape sequence specifying either an RGB color, or a user-customizable 8-bit ANSI
@@ -229,11 +243,11 @@ pub fn paint_text_foreground(text: &str, color: Color) -> String {
 // See
 // https://en.wikipedia.org/wiki/ANSI_escape_code#8-bit
 // https://github.com/ogham/rust-ansi-term/blob/ff7eba98d55ad609c7fcc8c7bb0859b37c7545cc/src/ansi.rs#L82-L112
-fn get_color_escape_sequence(color: Color, foreground: bool) -> String {
+fn get_color_escape_sequence(color: Color, foreground: bool, true_color: bool) -> String {
     if color.a == 0 {
         // See https://github.com/sharkdp/bat/pull/543
         format!("\x1b[{};5;{}m", if foreground { 38 } else { 48 }, color.r)
-    } else {
+    } else if true_color {
         format!(
             "\x1b[{};2;{};{};{}m",
             if foreground { 38 } else { 48 },
@@ -241,6 +255,12 @@ fn get_color_escape_sequence(color: Color, foreground: bool) -> String {
             color.g,
             color.b
         )
+    } else {
+        format!(
+            "\x1b[{};5;{}m",
+            if foreground { 38 } else { 48 },
+            ansi_colours::ansi256_from_rgb((color.r, color.g, color.b))
+        )
     }
 }
 
diff --git a/src/style.rs b/src/style.rs
index ae483c8f..80256805 100644
--- a/src/style.rs
+++ b/src/style.rs
@@ -19,62 +19,154 @@ pub fn is_no_syntax_highlighting_theme_name(theme_name: &str) -> bool {
     theme_name.to_lowercase() == "none"
 }
 
-pub const LIGHT_THEME_MINUS_COLOR: Color = Color {
+pub fn get_minus_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_MINUS_COLOR,
+        (true, false) => LIGHT_THEME_MINUS_COLOR_256,
+        (false, true) => DARK_THEME_MINUS_COLOR,
+        (false, false) => DARK_THEME_MINUS_COLOR_256,
+    }
+}
+
+pub fn get_minus_emph_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_MINUS_EMPH_COLOR,
+        (true, false) => LIGHT_THEME_MINUS_EMPH_COLOR_256,
+        (false, true) => DARK_THEME_MINUS_EMPH_COLOR,
+        (false, false) => DARK_THEME_MINUS_EMPH_COLOR_256,
+    }
+}
+
+pub fn get_plus_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_PLUS_COLOR,
+        (true, false) => LIGHT_THEME_PLUS_COLOR_256,
+        (false, true) => DARK_THEME_PLUS_COLOR,
+        (false, false) => DARK_THEME_PLUS_COLOR_256,
+    }
+}
+
+pub fn get_plus_emph_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_PLUS_EMPH_COLOR,
+        (true, false) => LIGHT_THEME_PLUS_EMPH_COLOR_256,
+        (false, true) => DARK_THEME_PLUS_EMPH_COLOR,
+        (false, false) => DARK_THEME_PLUS_EMPH_COLOR_256,
+    }
+}
+
+const LIGHT_THEME_MINUS_COLOR: Color = Color {
     r: 0xff,
     g: 0xe0,
     b: 0xe0,
     a: 0xff,
 };
 
-pub const LIGHT_THEME_MINUS_EMPH_COLOR: Color = Color {
+const LIGHT_THEME_MINUS_COLOR_256: Color = Color {
+    r: 224,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const LIGHT_THEME_MINUS_EMPH_COLOR: Color = Color {
     r: 0xff,
     g: 0xc0,
     b: 0xc0,
     a: 0xff,
 };
 
-pub const LIGHT_THEME_PLUS_COLOR: Color = Color {
+const LIGHT_THEME_MINUS_EMPH_COLOR_256: Color = Color {
+    r: 217,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const LIGHT_THEME_PLUS_COLOR: Color = Color {
     r: 0xd0,
     g: 0xff,
     b: 0xd0,
     a: 0xff,
 };
 
-pub const LIGHT_THEME_PLUS_EMPH_COLOR: Color = Color {
+const LIGHT_THEME_PLUS_COLOR_256: Color = Color {
+    r: 194,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const LIGHT_THEME_PLUS_EMPH_COLOR: Color = Color {
     r: 0xa0,
     g: 0xef,
     b: 0xa0,
     a: 0xff,
 };
 
-pub const DARK_THEME_MINUS_COLOR: Color = Color {
+const LIGHT_THEME_PLUS_EMPH_COLOR_256: Color = Color {
+    r: 157,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_MINUS_COLOR: Color = Color {
     r: 0x3f,
     g: 0x00,
     b: 0x01,
     a: 0xff,
 };
 
-pub const DARK_THEME_MINUS_EMPH_COLOR: Color = Color {
+const DARK_THEME_MINUS_COLOR_256: Color = Color {
+    r: 52,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_MINUS_EMPH_COLOR: Color = Color {
     r: 0x90,
     g: 0x10,
     b: 0x11,
     a: 0xff,
 };
 
-pub const DARK_THEME_PLUS_COLOR: Color = Color {
+const DARK_THEME_MINUS_EMPH_COLOR_256: Color = Color {
+    r: 124,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_PLUS_COLOR: Color = Color {
     r: 0x00,
     g: 0x28,
     b: 0x00,
     a: 0xff,
 };
 
-pub const DARK_THEME_PLUS_EMPH_COLOR: Color = Color {
+const DARK_THEME_PLUS_COLOR_256: Color = Color {
+    r: 22,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_PLUS_EMPH_COLOR: Color = Color {
     r: 0x00,
     g: 0x60,
     b: 0x00,
     a: 0xff,
 };
 
+const DARK_THEME_PLUS_EMPH_COLOR_256: Color = Color {
+    r: 28,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
 /// A special color to specify that no color escape codes should be emitted.
 pub const NO_COLOR: Color = Color::BLACK;