Add theme customizer.

build.zig

@@ -85,6 +85,36 @@ pub fn build(b: *std.Build) void {
     const test_step = b.step("test", "Run the tests");
     test_step.dependOn(&b.addRunArtifact(tests).step);
 
+    // The palette is the third root: a table of colours, the rules for
+    // resolving one, and the CSS it is written out as. It reaches libghostty-vt
+    // for the terminal's own default palette and nothing else, so it tests
+    // without a display in the same way the two above do.
+    const palette_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/palette.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
+    });
+    palette_tests.root_module.addImport("ghostty-vt", ghostty.module("ghostty-vt"));
+    test_step.dependOn(&b.addRunArtifact(palette_tests).step);
+
+    // And a root for the settings file itself. `Settings.zig` reaches GLib for
+    // where the file lives, but reading and writing its *contents* is pure
+    // enough to test — which is what these cover, since the palette overrides
+    // are a nested object whose shape one launch has to agree with the next on.
+    const settings_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/Settings.zig"),
+            .target = target,
+            .optimize = optimize,
+            .link_libc = true,
+        }),
+    });
+    settings_tests.root_module.addImport("glib", gobject.module("glib2"));
+    settings_tests.root_module.addImport("ghostty-vt", ghostty.module("ghostty-vt"));
+    test_step.dependOn(&b.addRunArtifact(settings_tests).step);
+
     // A second root for the same reason, one step further out: `emoji.zig` is a
     // table and a search over it, and it imports nothing at all. It cannot hang
     // off the root above because a test binary has exactly one root, and