Theme · Editing: a theme laid open, and an owner's palette per mode

settings::ThemeEditor lays a theme open: density, corners, text size,
high contrast and reduced motion, then twelve colors of the mode shown,
each a swatch that opens a picker beside its hex; each change hands the
owner a whole draft. Theme::set_palette takes an owner's palette per
mode, the colors fading to it when the mode shows; Theme::palette names
the palette the shown mode fades to; Palette::token_mut and
Syntax::token_mut reach a color by its field's name. The gallery's
theme page applies the editor to the whole gallery live, with a way
back to Ely's theme.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/theme/tests.rs

@@ -0,0 +1,35 @@
+use gpui::{TestAppContext, hsla};
+
+use super::{ActiveTheme, Mode, Palette, Theme};
+
+/// An owner's palette serves its own mode; the other keeps Ely's, and none brings Ely's back.
+#[gpui::test]
+fn an_owners_palette_serves_its_mode_alone(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let mut own = Palette::light(false);
+    own.accent = hsla(0.6, 0.5, 0.5, 1.0);
+    cx.update(|cx| Theme::set_palette(Mode::Light, Some(own.clone()), cx));
+    assert_eq!(cx.read(|cx| cx.theme().palette()), own);
+    cx.update(|cx| Theme::set_mode(Mode::Dark, cx));
+    assert_eq!(cx.read(|cx| cx.theme().palette()), Palette::dark(false));
+    cx.update(|cx| {
+        Theme::set_mode(Mode::Light, cx);
+        Theme::set_palette(Mode::Light, None, cx);
+    });
+    assert_eq!(cx.read(|cx| cx.theme().palette()), Palette::light(false));
+}
+
+#[test]
+fn every_named_color_answers() {
+    let mut palette = Palette::light(false);
+    *palette.token_mut("accent") = hsla(0.1, 0.2, 0.3, 1.0);
+    assert_eq!(palette.accent, hsla(0.1, 0.2, 0.3, 1.0));
+    *palette.syntax.token_mut("comment") = hsla(0.4, 0.2, 0.3, 1.0);
+    assert_eq!(palette.syntax.comment, hsla(0.4, 0.2, 0.3, 1.0));
+}
+
+#[test]
+#[should_panic(expected = "no palette color glow")]
+fn an_unknown_color_fails() {
+    Palette::light(false).token_mut("glow");
+}