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CSS Variable Theme Builder - Online Light & Dark Mode

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CSS Variable Theme Builder

Design light & dark mode themes with CSS custom properties — live preview, instant export

Default Ocean Forest Sunset Plum Mono
Editing: Light Mode
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Frequently Asked Questions

CSS custom properties (also called CSS variables) are reusable values defined with a -- prefix. They allow you to store colors, spacing, fonts, and other design tokens in one place. For theming, they're ideal because you can define different values for [data-theme="light"] and [data-theme="dark"] selectors, enabling instant theme switching without JavaScript overhead. They're natively supported in all modern browsers and cascade naturally through the DOM.

Simply add the generated CSS to your stylesheet, then toggle the data-theme attribute on your <html> or <body> element. You can use JavaScript: document.documentElement.setAttribute('data-theme', 'dark') to switch themes. For persistence, save the user's preference in localStorage and apply it on page load. The CSS cascade handles the rest — all elements using var(--your-variable) will update automatically.

Absolutely! Bootstrap 5 uses CSS variables extensively (e.g., --bs-primary, --bs-body-bg). You can override Bootstrap's default variables with your own theme values. For Tailwind CSS, you can reference CSS variables in your tailwind.config.js or use them directly in utility classes via arbitrary values. This builder exports standard CSS variables that work seamlessly with any framework.

Store the preference in localStorage:
localStorage.setItem('theme', 'dark')
On page load, read it back and apply it before the page renders to avoid a flash of wrong theme (FOIT). You can also check the user's OS preference via window.matchMedia('(prefers-color-scheme: dark)') as a default, then let them override it. This builder's output works perfectly with such a setup.

Key practices include: (1) Maintain sufficient contrast ratios — at least 4.5:1 for body text and 3:1 for large text per WCAG AA. (2) Avoid pure black (#000) backgrounds — use dark grays like #1a1a2e to reduce eye strain. (3) Don't just invert colors — adjust saturation and brightness thoughtfully. (4) Ensure focus indicators remain visible. (5) Test with real content. This builder's presets follow these principles to give you a solid starting point.

CSS custom properties are supported in all modern browsers: Chrome 49+, Firefox 31+, Safari 9.1+, Edge 15+, and Opera 36+. Internet Explorer does not support them. Global usage is approximately 97%+ as of 2024. If you need IE11 support, consider using a PostCSS plugin or a CSS variable polyfill, though most projects have dropped IE11 support entirely.

A minimal theme can work with just 6–8 variables (background, text, accent, border). For a production-ready design system, 10–16 variables provide good coverage — including backgrounds (2–3 levels), text (2–3 levels), accents, borders, shadows, and semantic colors (success, error, warning). This builder provides 12 carefully chosen variables that balance flexibility with simplicity, suitable for most web applications and landing pages.
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