CSS Custom Properties (Variables) Best Practices
CSS custom properties enable dynamic theming, design tokens, and maintainable style systems. Learn how to organize, scope, and use CSS variables effectively in production applications.
Key Takeaways
- CSS custom properties (often called CSS variables) are named values that you define and reuse throughout your stylesheets.
- Define variables with `--` prefix on any element.
- Adopt a consistent naming scheme from the start:
- Custom properties make dark mode implementation clean:
- Custom properties trigger style recalculation when changed.
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What Are CSS Custom Properties?
CSS custom properties (often called CSS variables) are named values that you define and reuse throughout your stylesheets. Unlike preprocessor variables (Sass/Less), custom properties are live in the browser and can be changed at runtime with JavaScript.
Defining and Using Variables
Define variables with -- prefix on any element. The :root selector is conventional for global variables:
:root {
--color-primary: #3B82F6;
--spacing-md: 1rem;
--font-size-base: 1rem;
}
Reference with var() and optionally provide a fallback:
color: var(--color-primary, blue);
Naming Conventions
Adopt a consistent naming scheme from the start:
- Semantic names:
--color-primary,--color-danger(what it means). - Descriptive names:
--blue-500,--red-600(what it looks like). - Two-tier system: Map descriptive to semantic for maximum flexibility.
Dark Mode with Custom Properties
Custom properties make dark mode implementation clean:
:root { --bg: #ffffff; --text: #1a1a1a; }
.dark { --bg: #0f172a; --text: #f1f5f9; }
All components using these variables automatically adapt when the .dark class is toggled.
Performance Considerations
Custom properties trigger style recalculation when changed. Avoid changing many variables simultaneously during animations. For animated properties, consider using @property to enable GPU-accelerated transitions.
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