Autodetect dark/light terminals

Closes #890

CHANGELOG.md

@@ -10,6 +10,9 @@ Difftastic is now smarter about calculating the display width for
 side-by-side diffs. Long lines that are not included in the output no
 longer affect display.
 
+The `--background` option now supports `auto` to automatically detect
+the terminal background color.
+
 ## 0.65 (released 23rd September 2025)
 
 ### Build

Cargo.lock

@@ -273,6 +273,7 @@ dependencies = [
  "streaming-iterator",
  "strsim",
  "strum",
+ "terminal-colorsaurus",
  "tikv-jemallocator",
  "tree-sitter",
  "tree-sitter-bash",
@@ -964,6 +965,32 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "terminal-colorsaurus"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f7226dad4b1817567c1e2f5d453897ef36abe79def7783af3fa241a694e30b3"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "memchr",
+ "mio",
+ "terminal-trx",
+ "windows-sys 0.59.0",
+ "xterm-color",
+]
+
+[[package]]
+name = "terminal-trx"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "975b4233aefa1b02456d5e53b22c61653c743e308c51cf4181191d8ce41753ab"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "windows-sys 0.59.0",
+]
+
 [[package]]
 name = "terminal_size"
 version = "0.4.1"
@@ -1674,6 +1701,12 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8e3e6735fcde06432870db8dc9d7e3ab1b93727c14eaef329969426299f28893"
 
+[[package]]
+name = "xterm-color"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4de5f056fb9dc8b7908754867544e26145767187aaac5a98495e88ad7cb8a80f"
+
 [[package]]
 name = "yansi"
 version = "1.0.1"

Cargo.toml

@@ -60,6 +60,7 @@ tree_magic_mini = "3.1.6"
 bumpalo = "3.16.0"
 unicode-width = "0.1.9"
 crossterm = { version = "0.28.0", features = ["windows"] }
+terminal-colorsaurus = "1.0"
 glob = "0.3.1"
 strum = { version = "0.26", features = ["derive"] }
 hashbrown = "0.14.0"

src/options.rs

@@ -201,8 +201,8 @@ json: Output the results as a machine-readable JSON array with an element per fi
             Arg::new("background").long("background")
                 .value_name("BACKGROUND")
                 .env("DFT_BACKGROUND")
-                .value_parser(["dark", "light"])
+                .value_parser(["dark", "light", "auto"])
-                .default_value("dark")
+                .default_value("auto")
                 .action(ArgAction::Set)
                 .help("Set the background brightness. Difftastic will prefer brighter colours on dark backgrounds.")
         )
@@ -782,6 +782,7 @@ pub(crate) fn parse_args() -> Mode {
     {
         "dark" => BackgroundColor::Dark,
         "light" => BackgroundColor::Light,
+        "auto" => detect_background_color().unwrap_or(BackgroundColor::Dark),
         _ => unreachable!("clap has already validated the values"),
     };
 
@@ -960,6 +961,15 @@ pub(crate) fn parse_args() -> Mode {
     }
 }
 
+/// Try to detect the terminal background color.
+fn detect_background_color() -> Option<BackgroundColor> {
+    match terminal_colorsaurus::theme_mode(terminal_colorsaurus::QueryOptions::default()) {
+        Ok(terminal_colorsaurus::ThemeMode::Dark) => Some(BackgroundColor::Dark),
+        Ok(terminal_colorsaurus::ThemeMode::Light) => Some(BackgroundColor::Light),
+        Err(_) => None,
+    }
+}
+
 /// Try to work out the width of the terminal we're on, or fall back
 /// to a sensible default value.
 fn detect_terminal_width() -> usize {