Tailwind CSS Academy · レッスン

CSS 変数ベースのテーマ設定

デザイントークンを CSS カスタムプロパティとして保存し、Tailwind の設定から参照することで、再コンパイルなしで実行時にテーマを切り替えられるようにします。

「CSS 変数ベースのテーマ設定」はCoddyKit上の無料Tailwind CSS Academyレッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはTailwind CSS Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Tailwind CSS Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Why CSS Variables for Theming?

CSS custom properties (CSS variables) are the key to runtime theming with Tailwind. Unlike static values baked into the compiled CSS, CSS variables can be changed by JavaScript at any time without reloading the page. This makes them ideal for implementing light/dark mode switches, user-selectable themes, or multi-tenant branding where the same application needs to look different for different clients.

Defining CSS Variables in :root

CSS variables are defined on the :root selector so they are globally available throughout the page. The syntax is --variable-name: value. You reference them with var(--variable-name). Place these definitions in your global CSS file — typically globals.css or index.css — so they are present before any component styles are applied.

/* globals.css */
@tailwind base;
@tailwind components;
@tailwind utilities;

@layer base {
  :root {
    --color-primary: #3b82f6;
    --color-primary-hover: #1d4ed8;
    --color-surface: #ffffff;
    --color-surface-alt: #f8fafc;
    --color-text: #111827;
    --color-text-muted: #6b7280;
    --color-border: #e5e7eb;
  }
}

Wiring CSS Variables Into Tailwind Config

Once your CSS variables are defined, you reference them in tailwind.config.js using the var(--variable-name) syntax. Tailwind will generate utility classes that use these variables as their values. When the variable changes at runtime, every generated class that references it updates automatically — no recompilation needed.

// tailwind.config.js
module.exports = {
  theme: {
    extend: {
      colors: {
        primary: 'var(--color-primary)',
        'primary-hover': 'var(--color-primary-hover)',
        surface: 'var(--color-surface)',
        'surface-alt': 'var(--color-surface-alt)',
        'text-base': 'var(--color-text)',
        'text-muted': 'var(--color-text-muted)',
        border: 'var(--color-border)'
      }
    }
  }
}

Using Variable-Backed Classes in HTML

After wiring variables to Tailwind, your utility classes work exactly as before — the only difference is that the underlying value is now dynamic. Writing bg-surface applies background-color: var(--color-surface) to the element. Switch the variable value in CSS and every element using bg-surface updates without any HTML or JavaScript changes.

<div class="bg-surface text-text-base border border-border rounded-xl p-6">
  <h2 class="text-xl font-semibold text-primary mb-2">Card Title</h2>
  <p class="text-text-muted text-sm">This card adapts to any theme.</p>
  <button class="bg-primary hover:bg-primary-hover text-white px-4 py-2 rounded-lg mt-4">
    Action
  </button>
</div>

Dark Mode With CSS Variables

To implement dark mode, you override the CSS variable values under a .dark class on the root element. When JavaScript toggles the dark class on html, all variable values switch simultaneously, instantly updating every component. This approach requires zero additional Tailwind utilities beyond the initial variable-backed classes you already have.

@layer base {
  :root {
    --color-primary: #3b82f6;
    --color-surface: #ffffff;
    --color-text: #111827;
  }

  .dark {
    --color-primary: #60a5fa;
    --color-surface: #1e293b;
    --color-text: #f1f5f9;
  }
}

Opacity Support With CSS Variables

There is a subtlety with CSS variable colors and Tailwind's opacity modifier syntax. Utilities like bg-primary/50 require the color to be in RGB channel format so Tailwind can inject the alpha value. To support opacity modifiers, define your variables as RGB triplets without the rgb() wrapper and use the rgb(var() / alpha) pattern in the config.

/* globals.css — RGB triplet format */
:root {
  --color-primary-rgb: 59 130 246;
}

/* tailwind.config.js */
module.exports = {
  theme: {
    extend: {
      colors: {
        primary: ({
          opacityValue
        }) => opacityValue
          ? 'rgb(var(--color-primary-rgb) / ' + opacityValue + ')'
          : 'rgb(var(--color-primary-rgb))'
      }
    }
  }
}

Multiple Theme Variants

CSS variables allow you to define multiple complete themes beyond just light and dark. Using data-theme attributes, you can scope different variable sets to different parts of the page or switch the entire application theme. This is particularly useful for multi-tenant SaaS applications where each customer brand needs its own color scheme from a shared codebase.

@layer base {
  [data-theme='ocean'] {
    --color-primary: #0ea5e9;
    --color-surface: #0c4a6e;
    --color-text: #e0f2fe;
  }

  [data-theme='forest'] {
    --color-primary: #16a34a;
    --color-surface: '#dcfce7';
    --color-text: #14532d;
  }
}

<!-- Switch theme with JS -->
document.documentElement.setAttribute('data-theme', 'ocean');

Animating Theme Transitions

When switching themes, a sudden color change can feel jarring. Adding a CSS transition on all properties to the root element creates a smooth color morph effect as variables change. Be selective — transitioning too many properties or using long durations can make the UI feel sluggish. A 150-300ms transition on color-related properties is typically ideal.

/* globals.css */
*, *::before, *::after {
  transition-property: color, background-color, border-color;
  transition-duration: 200ms;
  transition-timing-function: ease;
}

/* Exclude transitions from elements that should be instant */
.no-theme-transition,
.no-theme-transition * {
  transition: none !important;
}

Persisting Theme Preference

A theme switch is useless if it resets on page reload. Use localStorage to persist the user's preference and restore it on the next visit. The restoration script must run before the page renders to avoid a flash of the wrong theme. Place an inline script at the top of your HTML head — not in a module that loads after painting.

<!-- In <head> before any CSS -->
<script>
  (function() {
    var theme = localStorage.getItem('theme') ||
      (window.matchMedia('(prefers-color-scheme: dark)').matches
        ? 'dark' : 'light');
    document.documentElement.setAttribute('data-theme', theme);
    if (theme === 'dark') {
      document.documentElement.classList.add('dark');
    }
  })();
</script>

Semantic Variable Naming Patterns

Use a consistent naming pattern for your CSS variables to make them predictable and self-documenting. A three-part pattern of --category-role-state works well. For example: --color-action-default, --color-action-hover, --color-surface-raised, --color-feedback-error. Avoid vague names like --color1 or overly specific names like --signup-button-blue.

:root {
  /* Action colors */
  --color-action-default: #3b82f6;
  --color-action-hover: #1d4ed8;
  --color-action-disabled: #93c5fd;

  /* Surface colors */
  --color-surface-base: #ffffff;
  --color-surface-raised: #f8fafc;
  --color-surface-overlay: #f1f5f9;

  /* Feedback colors */
  --color-feedback-success: #16a34a;
  --color-feedback-error: #dc2626;
  --color-feedback-warning: #d97706;
}

Debugging CSS Variable Themes

When a themed color is not appearing correctly, the browser DevTools are your best friend. The computed styles panel shows the resolved value of any CSS variable on any element. You can also inspect :root directly to see all variable values. If you are seeing the raw var(--name) string instead of a color, the variable is not defined in the current scope — check for typos or missing :root blocks.

/* Quick debug: visualize all theme colors */
.theme-debug {
  background-color: var(--color-surface-base);
  color: var(--color-text-primary);
  border: 2px solid var(--color-border);
}

/* In DevTools console: */
/* getComputedStyle(document.documentElement)
   .getPropertyValue('--color-primary') */

Quick Check

Test your understanding of Tailwind CSS Mastery concepts from this lesson.

Lesson Recap

In this lesson you learned: CSS custom properties defined on :root serve as dynamic design tokens, Tailwind's config can reference them with var(--name) so generated classes inherit runtime changeability, and multiple themes are implemented by swapping variable values under different class or data-attribute selectors. Next up we explore multi-brand theming strategies for product teams.

よくある質問

「CSS 変数ベースのテーマ設定」レッスンは無料ですか？

はい。「CSS 変数ベースのテーマ設定」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Tailwind CSS Academyコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Tailwind CSS Academyコースには全4レッスンが含まれています。

「CSS 変数ベースのテーマ設定」で何を学びますか？

デザイントークンを CSS カスタムプロパティとして保存し、Tailwind の設定から参照することで、再コンパイルなしで実行時にテーマを切り替えられるようにします。ブラウザで直接実行するハンズオンコードでTailwind CSS Academyを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Tailwind CSS Academyを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのTailwind CSS Academyは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン2/4です。

「CSS 変数ベースのテーマ設定」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このTailwind CSS Academyレッスンでコードを書いて実行できますか？

はい。すべてのTailwind CSS Academyレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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