```jsx
// .storybook/preview.jsx
import React from 'react';
import { ThemeProvider } from 'styled-components';

// themes = { light, dark }
import * as themes from '../src/themes';

export default {
  decorators: [
    // 👇 Defining the decorator in the preview file applies it to all stories
    (Story, { parameters }) => {
      // 👇 Make it configurable by reading the theme value from parameters
      const { theme = 'light' } = parameters;
      return (
        <ThemeProvider theme={themes[theme]}>
          <Story />
        </ThemeProvider>
      );
    },
  ],
};
```