storybook/docs/_snippets/storybook-preview-docs-dark-theme.md

import { themes } from '@storybook/theming';

export default {
  parameters: {
    docs: {
      theme: themes.dark,
    },
  },
};

// Replace your-framework with the framework you are using (e.g., react, vue3)
import { Preview } from '@storybook/your-framework';

import { themes } from '@storybook/theming';

const preview: Preview = {
  parameters: {
    docs: {
      theme: themes.dark,
    },
  },
};

export default preview;