Revert "Autodetect dark/light terminals"

This reverts commit bf335094b8.

Doing `git dlog -p` and waiting shows a bunch of

ESCESCESC

in the terminal, so I don't think terminal-colorsaurus is
quite ready for difftastic yet.

CHANGELOG.md

@@ -10,9 +10,6 @@ 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

@@ -273,7 +273,6 @@ dependencies = [
  "streaming-iterator",
  "strsim",
  "strum",
- "terminal-colorsaurus",
  "tikv-jemallocator",
  "tree-sitter",
  "tree-sitter-ada",
@@ -972,32 +971,6 @@ 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"
@@ -1778,12 +1751,6 @@ 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

@@ -60,7 +60,6 @@ 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

@@ -238,8 +238,8 @@ 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", "auto"])
-                .default_value("auto")
+                .value_parser(["dark", "light"])
+                .default_value("dark")
                 .action(ArgAction::Set)
                 .help("Set the background brightness. Difftastic will prefer brighter colours on dark backgrounds.")
         )
@@ -819,7 +819,6 @@ 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"),
     };
 
@@ -997,15 +996,6 @@ 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 {