Provide an environment variable for controlling background colour

Fixes #55

CHANGELOG.md

@@ -24,7 +24,7 @@ Added a `--color` option which allows explicitly enabling/disabling
 colour output.
 
 Added a `--background` option which controls whether difftastic uses
-bright or dark colours.
+bright or dark colours. This can also be controlled by `DFT_BACKGROUND`.
 
 ## 0.17 (released 25 January 2022)
 

src/main.rs

@@ -149,9 +149,8 @@ fn app() -> clap::App<'static> {
         )
         .arg(
             Arg::new("background").long("background")
-                .default_value("dark")
                 .possible_values(["dark", "light"])
-                .help("Set the background color. Difftastic will prefer brighter colours on dark backgrounds.")
+                .help("Set the background color. Overrides $DFT_BACKGROUND if present. Difftastic will prefer brighter colours on dark backgrounds.")
         )
         .arg(
             Arg::new("paths")
@@ -244,8 +243,16 @@ fn parse_args() -> Mode {
         } else {
             BackgroundColor::Dark
         }
+    } else {
+        if let Ok(background) = env::var("DFT_BACKGROUND") {
+            if background == "light" {
+                BackgroundColor::Light
             } else {
                 BackgroundColor::Dark
+            }
+        } else {
+            BackgroundColor::Dark
+        }
     };
 
     Mode::Diff {