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.

src/settings/mod.rs

@@ -1,5 +1,6 @@
 mod advanced;
 mod appearance;
+mod editor;
 mod network;
 mod preferences;
 mod search;
@@ -14,6 +15,7 @@ pub use advanced::{DeveloperModeToggle, FeatureFlags, Flag, Stage};
 pub use appearance::{
     AccentColorPicker, Appearance, DensitySelector, FontSizeControl, ThemeSelector,
 };
+pub use editor::{ThemeDraft, ThemeEditor};
 pub use network::{Proxy, ProxySettings, manual};
 pub use preferences::{
     Notices, NotificationSettings, Privacy, PrivacySettings, Startup, StartupSettings,