The-Notorious-Avengers Dark Mode Theming Weapon
The dark-mode theming surface is one of the most deceptively complex areas of a modern React/Next.js stack. Getting it right requires aligning four systems: CSS token architecture, a theme provider library, SSR hydration semantics, and (in multi-brand apps) a runtime CSS variable override layer. Getting any one of them wrong produces visible flashes, hydration warnings, inaccessible OS-native controls, or cross-tenant token leakage.
This Weapon encodes the 2026 consensus patterns for all of these. Read SKILL.md for the overview and task routing; follow the specific guide for deep implementation.
Task routing
| Task | Guide |
|---|---|
| Understand principles and token contract | guides/00-principles.md |
Build the :root / .dark CSS variable layer |
guides/01-css-token-architecture.md |
Wire next-themes ThemeProvider |
guides/02-next-themes-wiring.md |
| Eliminate flash-of-wrong-theme | guides/03-fowt-prevention.md |
| Fix SSR hydration warnings / mounted guard | guides/04-ssr-hydration-safety.md |
| Configure Tailwind v4 dark mode | guides/05-tailwind-v4-dark-mode.md |
| Implement multi-brand / white-label theming | guides/06-multi-brand-runtime-swap.md |
| Happy-path example (App Router + next-themes + Tailwind v4) | examples/happy-path-app-router.md |
| Edge-case example (cookie-based SSR match) | examples/edge-case-cookie-ssr.md |
| Token layer skeleton template | templates/tokens.css.template.md |
| Audit report template | templates/audit-report.template.md |
The six non-negotiables
These are the directives from the Command Brief, repeated here as the guardrails for every implementation:
- Never emit raw hex in component code. All color references go through
var(--token-name). Seeguides/00-principles.md. - Always inject the FOWT-prevention script before first paint. See
guides/03-fowt-prevention.md. - Distinguish
prefers-color-scheme(system preference) from persisted preference (localStorage / cookie). System preference is the fallback, not the override. Seeguides/02-next-themes-wiring.md. - Flag
typeof windowguards in every SSR-executed code path that reads theme state. Seeguides/04-ssr-hydration-safety.md. - Scope multi-brand overrides to CSS variables, not JS state. CSS variable overrides are zero-JS and zero-rerender. See
guides/06-multi-brand-runtime-swap.md. - Separate semantic tokens from primitive tokens.
--color-primarymust point to a semantic alias, not a raw hex value. Seeguides/01-css-token-architecture.md.
Scope boundary
| In scope | Out of scope — route to |
|---|---|
CSS variable token layer (:root, .dark, per-brand blocks) |
Palette creation, token source-of-truth file — design-system-guardian |
next-themes wiring (ThemeProvider, props, storageKey) |
Per-component visual deltas (which token for which role) — ux-ui-guardian |
| FOWT prevention (inline script, script placement) | Persisted-preference DB schema (user_preferences.theme) — db-guardian |
SSR hydration safety (suppressHydrationWarning, guards) |
CSS variable injection input validation — security-guardian |
Tailwind v4 dark mode (@custom-variant) |
Tailwind config beyond darkMode — ux-ui-guardian |
| Multi-brand CSS variable overrides | Auth-gated per-user theme (server-side preference + RBAC) — auth-guardian + db-guardian |
Quick-start checklist
For a standard Next.js 15 App Router + next-themes + Tailwind v4 stack:
- Token layer:
:rootsemantic tokens +.darkoverrides inglobals.css -
color-schemeCSS property set on:rootand.dark -
meta[name="color-scheme"]inlayout.tsx -
ThemeProviderin a"use client"providers component -
suppressHydrationWarningon<html> -
disableTransitionOnChangeenabled onThemeProvider - Tailwind:
@custom-variant dark (&:where(.dark, .dark *))inglobals.css - No raw hex values in any component
.tsxfile
Research trail: research/research-summary.md | Command Brief: ai-tools/command-briefs/dark-mode-theming-guardian-command-brief.md