config: add window-theme = auto for automatic choosing based on bg color

src/apprt/gtk/App.zig

@@ -133,6 +133,14 @@ pub fn init(core_app: *CoreApp, opts: Options) !App {
         c.adw_style_manager_set_color_scheme(
             style_manager,
             switch (config.@"window-theme") {
+                .auto => auto: {
+                    const lum = config.background.toTerminalRGB().perceivedLuminance();
+                    break :auto if (lum > 0.5)
+                        c.ADW_COLOR_SCHEME_PREFER_LIGHT
+                    else
+                        c.ADW_COLOR_SCHEME_PREFER_DARK;
+                },
+
                 .system => c.ADW_COLOR_SCHEME_PREFER_LIGHT,
                 .dark => c.ADW_COLOR_SCHEME_FORCE_DARK,
                 .light => c.ADW_COLOR_SCHEME_FORCE_LIGHT,

src/config/Config.zig

@@ -600,9 +600,13 @@ keybind: Keybinds = .{},
 /// borders.
 @"window-decoration": bool = true,
 
-/// The theme to use for the windows. The default is `system` which means that
-/// whatever the system theme is will be used. This can also be set to `light`
-/// or `dark` to force a specific theme regardless of the system settings.
+/// The theme to use for the windows. Valid values:
+///
+///   * `auto` - Determine the theme based on the configured terminal
+///      background color.
+///   * `system` - Use the system theme.
+///   * `light` - Use the light theme regardless of system theme.
+///   * `dark` - Use the dark theme regardless of system theme.
 ///
 /// On macOS, if `macos-titlebar-tabs` is set, the window theme will be
 /// automatically set based on the luminosity of the terminal background color.
@@ -610,7 +614,7 @@ keybind: Keybinds = .{},
 /// non-terminal windows within Ghostty.
 ///
 /// This is currently only supported on macOS and Linux.
-@"window-theme": WindowTheme = .system,
+@"window-theme": WindowTheme = .auto,
 
 /// The colorspace to use for the terminal window. The default is `srgb` but
 /// this can also be set to `display-p3` to use the Display P3 colorspace.
@@ -3342,6 +3346,7 @@ pub const OSCColorReportFormat = enum {
 
 /// The default window theme.
 pub const WindowTheme = enum {
+    auto,
     system,
     light,
     dark,

src/terminal/color.zig

@@ -144,6 +144,17 @@ pub const RGB = struct {
         return std.math.pow(f64, (normalized + 0.055) / 1.055, 2.4);
     }
 
+    /// Calculates "perceived luminance" which is better for determining
+    /// light vs dark.
+    ///
+    /// Source: https://www.w3.org/TR/AERT/#color-contrast
+    pub fn perceivedLuminance(self: RGB) f64 {
+        const r_f64: f64 = @floatFromInt(self.r);
+        const g_f64: f64 = @floatFromInt(self.g);
+        const b_f64: f64 = @floatFromInt(self.b);
+        return 0.299 * (r_f64 / 255) + 0.587 * (g_f64 / 255) + 0.114 * (b_f64 / 255);
+    }
+
     test "size" {
         try std.testing.expectEqual(@as(usize, 24), @bitSizeOf(RGB));
         try std.testing.expectEqual(@as(usize, 3), @sizeOf(RGB));