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.

TASKS.md

@@ -132,7 +132,9 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T37b Onboarding · Help — `tasks/ch31-43.md`
 - [x] T38a Interaction · Pointer — `tasks/ch31-43.md`
 - [x] T38b Interaction · Keys, scroll and focus — `tasks/ch31-43.md`
-- [ ] T39 Theme
+- [ ] T39a Theme · Editing
+- [ ] T39b Theme · Importing
+- [ ] T39c Theme · Platform
 - [ ] T40 i18n & a11y
 - [ ] T41 Maps
 - [ ] T42 Misc