Partition the ~19 board/global theme colors into flat / clear / dark /
special and define where custom colors are allowed (flat = 1 accent,
clear = 2 for a color slide, dark/special = fixed none). This is the shared
source of truth for the redesigned Select Color picker (two-level dropdowns
+ color wheel) used by both Board Settings and Member Settings.
- models/lib/themeCategories.js: THEME_CATEGORIES + categoryOf /
colorsInCategory / allowsCustomColor / customColorCount /
isValidCustomColors (validates category + count + #rrggbb hex).
- tests/themeCategories.test.cjs (wired into test:unit:node): behaviour +
negatives + a source guard that the partition exactly equals
config/const.js ALLOWED_BOARD_COLORS.
- docs/Theme/Theme.md: full design — categories, two-level dropdown UX,
custom-color rules, CSS-variable rendering plan, storage
(board.customThemeColors / profile.globalThemeCustomColors), and
server-side validation.
Completes the global per-user theme override (Member Menu / Change Color)
begun in the previous commit.
Fixes#5778.
