chromaui/chromatic-skills/chromatic-themes

Chromatic Themes

Configure multi-theme visual testing using Chromatic's Modes API and @storybook/addon-themes.

This skill covers:

• Detecting the user's theme library and configuring the right decorator
• Defining theme modes in .storybook/modes.ts
• Applying modes at project, component, and story levels

Quick start

1. Scan the project for existing theme config: look for @storybook/addon-themes imports, a modes.ts file, or chromatic.modes.
2. Ask the user which theme library they use (see step 3 below).
3. Fetch the relevant docs from reference/docs-map.md.
4. Generate configuration files and output the checklist from reference/output-contract.md.

Required workflow

1) Detect existing configuration

Check .storybook/preview.ts, story files, and any modes.ts:

• withThemeByClassName or withThemeByDataAttribute found → addon already configured, help extend or wire up modes
• chromatic.modes with theme keys found → already on Modes API, help extend or fix
• Neither found → fresh setup, ask the user which theme library they use

2) Fetch current docs

Use the URLs in reference/docs-map.md and fetch with WebFetch. Use fetched content as the authoritative source — do not rely on remembered examples.

3) Install @storybook/addon-themes (fresh setup only)

If @storybook/addon-themes is not already installed, detect the package manager by checking for lock files (bun.lockb/bun.lock → bun, pnpm-lock.yaml → pnpm, yarn.lock → yarn, package-lock.json → npm) and use the matching install command:

Then register it in .storybook/main.ts:

export default {
  addons: ['@storybook/addon-themes'],
};

4) Identify the theme library and configure the decorator

Ask: _"Which theme library or approach does your project use? (e.g. Tailwind, Bootstrap, Material UI, styled-components, Emotion, or plain CSS classes)"_

Use the answer to pick the right path:

Class-based (withThemeByClassName):

// .storybook/preview.ts
import { withThemeByClassName } from '@storybook/addon-themes';
export const decorators = [
  withThemeByClassName({
    themes: { light: 'light-theme', dark: 'dark-theme' },
    defaultTheme: 'light',
    // parentSelector: 'html', // default
  }),
];

Data attribute (withThemeByDataAttribute):

import { withThemeByDataAttribute } from '@storybook/addon-themes';
export const decorators = [
  withThemeByDataAttribute({
    themes: { light: 'light', dark: 'dark' },
    defaultTheme: 'light',
    attributeName: 'data-bs-theme', // default: 'data-theme'
  }),
];

Custom decorator — fetch the library-specific guide from reference/docs-map.md and follow it. Use pluckThemeFromContext and initializeThemeState from @storybook/addon-themes to bridge Storybook globals with the library's theme API.

4) Define theme modes

Create or update .storybook/modes.ts. The theme key must exactly match a key in the decorator's themes object:

export const allModes = {
  light: { theme: 'light' },
  dark: { theme: 'dark' },
} as const;

5) Apply modes to stories

Project-wide (.storybook/preview.ts):

import { allModes } from './modes';
const preview = {
  parameters: { chromatic: { modes: allModes } },
};
export default preview;

Per component or per story:

import { allModes } from '../.storybook/modes';
export default {
  title: 'Button',
  parameters: {
    chromatic: { modes: { light: allModes.light, dark: allModes.dark } },
  },
};

Disable inherited modes on a specific story:

parameters: { chromatic: { modes: { dark: { disable: true } } } }

6) Output checklist

Always render the checklist from reference/output-contract.md, then show the complete content of every file to create or modify.

Key constraints

• The theme value in a mode must exactly match a key in the decorator's themes object — a mismatch silently falls back to the default theme
• Mode names are tied to baselines — renaming a mode creates a new baseline and retires the old one
• Modes stack across project → component → story levels; lower levels add to higher ones, they do not replace them
• @storybook/addon-themes requires Storybook 7.x or later
• Only one decorator approach per project — do not mix withThemeByClassName and withThemeByDataAttribute

Boundaries

• Do not commit or push files — generate and show configuration only
• Do not suggest viewport or locale modes unless the user asks
• Do not install or suggest theme libraries — only wire up what already exists

References

• reference/docs-map.md
• reference/output-contract.md
• ../chromatic-setup-ci/reference/package-manager.md — lock file detection and install commands per package manager