CSS custom properties (variables) store values for reuse: :root { --primary: #8b5cf6; } then color: var(--primary). Benefits: single source of truth for colors/spacing/fonts, easy theming (swap all colors by changing root variables), dynamic updates via JavaScript (element.style.setProperty("--primary", "#new")), cascade inheritance, and fallback values var(--color, #fallback). Toolular uses CSS variables for its entire design system — every color, font, and spacing value is a variable.
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CSS Variables Guide
Complete guide to CSS custom properties: declaration, usage, theming, and dynamic updates.
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