style: update color variable definitions

[Chrisliao0806]

Description

Context

After rebasing release/26P3_dev onto main, the entire PwC brand color scheme appeared "dead" across the app — primary buttons, switches, sliders, progress bars, focus rings, and the chat submit button all lost their orange accent and rendered transparent/colorless.

Root cause

The upstream rebase window included a color-system overhaul (LibreChat-AI#14670, LibreChat-AI#14709, LibreChat-AI#14734) that migrated all theme CSS variables to bare R G B channel triplets, consumed as rgb(var(--x) / <alpha-value>).

Our PwC theming commits were replayed correctly during the rebase, and every consumer was migrated to the new convention:

• client/tailwind.config.cjs — 'brand-primary': 'rgb(var(--brand-primary) / <alpha-value>)'
• client/src/style.css — .btn-primary { background-color: rgb(var(--brand-primary)); }
• client/src/style.css — --surface-submit: var(--brand-primary); (both html and .dark blocks)

However, the variable definitions were left in the old pre-rebase format as complete HSL color values:

--brand-primary: hsl(var(--brand-primary-hsl));

Every consumer therefore resolved to rgb(hsl(18 98% 51%) / 1) — invalid CSS that browsers silently drop, which killed the brand accent everywhere at once.

Change

Single fix in client/src/style.css: redefine the --brand-primary trio as RGB triplets sourced from the PwC palette, matching the new system-wide convention.

--brand-primary: var(--pwc-orange-500);        /* 253 81 8  — #fd5108 */
--brand-primary-hover: var(--pwc-orange-600);  /* 224 71 5  — #e04705 */
--brand-primary-subtle: var(--pwc-orange-100); /* 255 236 189 — #ffecbd */

The --brand-primary-*-hsl channel variables are kept, as the shadcn --ring token still consumes them via hsl(var(--ring)).

Also audited during investigation (no further changes needed):

• The 13 component-level hunks dropped during the rebase replay (e.g. TwoFactorScreen, MCPConfigDialog, Fork, SkillToggle — originally bg-green-* → bg-brand-primary) are obsolete: upstream refactored those components onto semantic tokens (accent-primary, ring-primary, surface-submit), which already map to the PwC palette.
• pwc-orange-* utilities, dark-mode overrides, and client/index.html are all intact and consistent with the triplet convention.

Tests

• Compiled the client Tailwind CSS with the real config (npx tailwindcss -c tailwind.config.cjs -i src/style.css -o out.css) and verified the generated rules now form a valid chain:
  • .bg-brand-primary → background-color: rgb(var(--brand-primary) / var(--tw-bg-opacity))
  • --brand-primary: var(--pwc-orange-500) → resolves to rgb(253 81 8 / 1) (PwC orange)
  • alpha modifiers work again, e.g. .bg-brand-primary/20 → rgb(var(--brand-primary) / 0.2)
• Verified no remaining consumer expects --brand-primary as a complete color value (grep for non-rgb()-wrapped usages across client/src and packages/client/src).
• Visual check in the running app: primary/confirm buttons, switch checked state, slider, progress bar, chat submit button, and focus rings render in PwC orange again in both light and dark mode.

Instruction and/or command lines to reproduce your tests:

# CSS-only change — no package rebuild required
npm run frontend:dev   # HMR picks it up; or `cd client && npm run build` for the 3080 build

Then open the app and confirm the PwC orange accent on: login/confirm buttons, switches, sliders, progress bars, and the chat send button (light + dark mode).

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run one-shot tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.