Build a complete CSS theme with light and dark modes
This CSS theme builder turns a handful of brand colors into a full multi-theme system of CSS custom properties — background, surface, text, border, primary, and state tokens — wired up for both light and dark mode in a single, copy-ready stylesheet. Instead of scattering hex codes across components, you define each color once as a semantic token and let the cascade do the rest.
Custom properties are the right primitive for theming because, unlike preprocessor variables, their values are live and update every element that references them the moment you swap the theme. That makes runtime theme switching, system-preference detection, and per-component overrides essentially free.
What the CSS variables theme generator gives you
- A
:rootblock with light-mode tokens (--bg,--surface,--text,--text-muted,--border,--primary, and more) - A matching dark-mode block scoped to
[data-theme="dark"]and@media (prefers-color-scheme: dark) - Semantic naming so colors describe purpose, not appearance — no more
--bg-lightthat turns dark - Contrast-aware defaults so body text clears WCAG AA against every surface in both modes
How the output is structured
:root {
--bg: #ffffff;
--surface: #f8fafc;
--text: #0f172a;
--primary: #6366f1;
}
[data-theme="dark"] {
--bg: #0b1020;
--surface: #111827;
--text: #e5e7eb;
--primary: #818cf8;
}Toggle the theme by setting document.documentElement.dataset.theme, and pair it with the prefers-color-scheme media query so first-time visitors get the right mode before any script runs.
Pair it with the rest of your color workflow
A theme is only as solid as the colors feeding it. Pull a starting palette from the brand color extractor, build perceptually even ramps with the tint and shade scale generator, then verify every foreground/background pair in the contrast grid before locking the tokens. When you need framework-specific output instead of raw CSS, push the same palette through the design token exporter for SCSS, Tailwind, or W3C JSON.
Who this CSS theme builder is for
Designers documenting a system, front-end engineers wiring up dark mode for the first time, and teams migrating a legacy stylesheet to tokens. The output is plain CSS — no framework lock-in — so it drops cleanly into Next.js, Astro, Rails, Laravel, or a static HTML page.