How to make VitePress tell Shiki to use light-dark()?

[KTibow]

Is there an easy way to set Shiki configuration like defaultColor as a VitePress consumer?

[MILLERMARRU]

There's no direct passthrough for it, VitePress's markdown config only exposes theme, languages, languageAlias, and codeTransformers, defaultColor isn't one of the surfaced options (and codeTransformers wouldn't help here anyway, transformers reshape the HAST output, they don't touch top-level codeToHtml options like defaultColor).

The escape hatch that does work is markdown.config(md), the same hook VitePress's own docs use to override the fence renderer for other customizations (their i18n example replaces the copy-button label this way). You can use it to bypass VitePress's built-in fence rendering for code blocks and call Shiki yourself with defaultColor: false:

// .vitepress/config.ts
import { createHighlighter } from 'shiki'

export default {
  markdown: {
    theme: { light: 'github-light', dark: 'github-dark' },
    async config(md) {
      const highlighter = await createHighlighter({
        themes: ['github-light', 'github-dark'],
        langs: [], // langs get loaded lazily as needed, or list them explicitly
      })

      md.renderer.rules.fence = (tokens, idx) => {
        const token = tokens[idx]
        return highlighter.codeToHtml(token.content, {
          lang: token.info.trim() || 'text',
          themes: { light: 'github-light', dark: 'github-dark' },
          defaultColor: false,
        })
      }
    },
  },
}

With defaultColor: false, Shiki emits both themes' colors as CSS variables on each token instead of picking one as the "default" with the other behind a class toggle, which is what lets you drive it with light-dark() (or your own prefers-color-scheme/data-attribute setup) in your own stylesheet instead of VitePress's built-in .dark class switching.

Worth filing as a feature request too, a markdown.shikiOptions (or similar) passthrough that merges into the internal codeToHtml call would save the next person from having to reimplement the fence renderer just to flip one flag.