Express Dimidium as a runtime theme entry

[deanjstone]

What

Move the Dimidium palette out of apps/web/src/index.css as a hand-edited :root override, and express it as an entry in upstream's runtime theming system (html[data-theme-id], the --app-theme-* token family) so it is selectable from Settings → Appearance like any other theme.

Why

The 2026-09-05 upstream sync (#20) resolved 21 conflicts, and 6 of them were in index.css alone — every one caused by this fork hard-editing the base palette in a file upstream actively develops.

During the six-week gap upstream built a proper runtime theming system and expanded the token set with roles the fork's palette never defined (--toolbar-*, --message-*, --error-surface, --update-*, --placeholder). The sync resolution took upstream's structure whole and appended Dimidium as a default-palette override — correct and working, but it leaves the same conflict surface in place for next time.

Expressing Dimidium as a theme entry instead would make index.css conflict-free on future syncs, since the fork would no longer be editing upstream's palette declarations at all.

The value is realised at the next sync, so this is worth doing before then rather than after.

Notes

• Current state after chore: sync fork with upstream main (1295 commits) #20: Dimidium is appended last in index.css as a :root block with a @variant dark nested block, deliberately outranking upstream's :root by source order while still losing to html[data-theme-id]:not([data-theme-id=""]) on specificity.
• Only base role values are overridden; derived roles (--destructive from --error, --ring from --primary, etc.) follow automatically via var() resolution. A theme entry would need to supply the --app-theme-* inputs instead.
• Reference values are in that block: light #f5f3f1/#1a8e46, dark #141414/#37e57b.
• Original palette commit: d3257c98a.

[deanjstone]

Resolution

Done in #24, merged as eea8c2369.

Dimidium is now a built-in theme in packages/shared/src/themePalettes.ts, selectable in Settings → Appearance. apps/web/src/index.css no longer touches upstream's palette declarations at all — 57 lines removed — which was the point: that file was 6 of the 21 conflicts in the 1295-commit sync, and it should now contribute none.

What could not be moved. --info and --success stay in index.css. Upstream deliberately keeps success, info, provider and channel identity colors outside the themeable role set, so a theme entry cannot carry them. That 21-line residue is the floor, not an oversight.

Deliberately not the boot default. Dimidium is selected once per client rather than shipped as the default. Making it the default would have required adding ~60 lines to index.html's inlined boot palette table to avoid a first-paint flash — trading one conflict surface for another, in a file upstream also maintains.

Two bugs fixed on the way, neither in scope when this was filed:

1. Sidebar rendering. main was showing the sidebar in upstream's cool zinc, not Dimidium's warm card surface. The sync had dropped the dead [data-sidebar-version] blocks and --sidebar: var(--card) went with them. All nine sidebar roles are now explicit, so this restored something main had silently lost.
2. A live WCAG AA failure. Placeholder-on-input measured 4.07:1 light and 3.23:1 dark against a 4.5:1 floor. The CSS override was never gated on contrast, so this shipped unnoticed; the mobile suite caught it immediately once the palette became a theme. Now 4.60:1 in both.

Conventions worth knowing before adding another theme — both were discovered by breaking them:

• Built-in palettes must be canonical OKLCH (enforced by themePalette.test.ts). Convert with the repo's own toCanonicalThemeColor.
• Values must be opaque and concrete. No other built-in uses alpha, and these roles feed React Native tokens, which cannot parse color-mix()/var(). Translucent values silently produce NaN in contrast assertions rather than failing loudly.
• Adding an id to BUILT_IN_THEME_IDS widens the mobile Uniwind union — regenerate via vp run --filter @t3tools/mobile generate plus the uniwind generate-artifacts CLI for the .d.ts.

Follow-up for the next sync: the real test of this is whether index.css conflicts at all. Worth checking then.

[deanjstone]

Correction — the resolution above is superseded

The Resolution comment on this issue describes Dimidium becoming a built-in theme in packages/shared/src/themePalettes.ts. That approach has been reversed in #25, in favour of using stock T3 Code capability: Dimidium is now a custom theme imported through Settings → Appearance, from a file generated by the repo's own serializeThemeFile.

This issue's actual goal is still met, and more completely than before. The point was to stop the fork hand-editing upstream's palette in index.css, which was 6 of the 21 conflicts in the 1295-commit sync. That file still carries no palette. The difference is that the fork now carries zero theme code at all rather than a built-in — five of the six files in #25 return to matching upstream/main byte-for-byte.

So the outcome is better than what this issue asked for, by a route it didn't anticipate.

What carries forward from #24 regardless of the revert:

• The WCAG AA fix. Placeholder-on-input measured 4.07:1 light and 3.23:1 dark against a 4.5 floor — a real defect that had shipped unnoticed, because a CSS override is never contrast-gated. The exported theme file carries the corrected values (4.60:1 both), so the imported theme is the accessible version.
• The sidebar fix. main had been rendering the sidebar in upstream's cool zinc since the sync dropped the dead [data-sidebar-version] blocks, taking --sidebar: var(--card) with them. All nine sidebar roles are explicit in the theme file.
• The theme-system conventions learned along the way (canonical OKLCH, opaque-only, concrete colours, the mobile artefact regeneration) remain true for anyone who does add a built-in later.

Known cost of going stock: the React Native mobile app enumerates built-ins only and has no custom-theme support, so Dimidium is unavailable there. The iPhone PWA is unaffected — it runs the web bundle.

Residue that cannot be removed: --info/--success stay in index.css (21 lines). Upstream deliberately keeps success, info, provider and channel identity colours outside the themeable role set, so no imported theme can carry them.