How to Build a Dark Mode Theme with CSS
Switch themes for just one section—not your whole site.
In this tutorial I build a zero-dependency, section-scoped theme system for a hero block using hex-only CSS variables, a tiny accessible theme dock (tabs with keyboard support), and persistent selection via localStorage. No frameworks, no recompiles—just clean tokens and fast UI.
What you’ll learn.
Section-scoped theming with the data-theme attribute.
Hex-only design tokens for light, dark, brand, and mint.
An accessible tablist dock with arrow-key navigation and aria-selected.
Persistent theme using LocalStorage + system dark fallback.
Auto-contrast trick: set --dock-inactive to black on light and white on dark.
Motion & perf: reduced-motion safe floats, lightweight glow/grid layers.
5 Simple CSS Theming Tricks with Hex Tokens, CSS Variables, Dark Mode, How to Build a Dark Mode Theme with CSS
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Chapters ↓ (click to jump)
00:00 — Hook: why global theme systems feel heavy
00:30 — Plan: section-only theme switcher (no frameworks)
00:42 — HTML setup: data-theme on the hero
01:01 — Content structure: semantic hero bits
01:09 — CSS tokens: scope by [data-theme]
01:36 — Base vs theme blocks: same names, new values
02:06 — Advanced tokens: pre-blended “alpha” hexes
02:41 — JavaScript overview: init, apply, delegate
03:12 — Click flow: update attribute + save preference
03:29 — Mobile polish: safe-area insets, reduced motion
03:42 — Accessibility: tablist, tabs, color contrast, keys
03:56 — Scale up: more themes & independent sections
04:25 — Add a new theme: CSS block + auto-pickup
04:33 — Performance: variables over style mutations
04:49 — Pitfalls: FOUC, specificity, browser support
05:09 — Copy-paste checklist (implementation steps)
05:42 — Callouts: source + advanced series
06:02 — Outro
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