Repository: tailwindlabs/tailwindcss
File: packages/tailwindcss/src/index.ts (TypeScript)

Code before:
    // import an external theme file as a reference, which becomes `@media
    // theme(reference) { … }` when the `@import` is processed.
    if (node.selector.startsWith('@media theme(')) {
      let themeParams = node.selector.slice(13, -1)

      walk(node.nodes, (child) => {
        if (child.kind !== 'rule') {
          throw new Error(
            'Files imported with `@import "…" theme(…)` must only contain `@theme` blocks.',
          )
        }
        if (child.selector === '@theme') {
          child.selector = '@theme ' + themeParams
          return WalkAction.Skip
        }
      })
      replaceWith(node.nodes)
      return WalkAction.Skip
    }

    if (node.selector !== '@theme' && !node.selector.startsWith('@theme ')) return

    let [_, ...themeOptions] = segment(node.selector, ' ')

    let isReference = themeOptions.includes('reference')
    let isInline = themeOptions.includes('inline')

    // Record all custom properties in the `@theme` declaration
    walk(node.nodes, (child, { replaceWith }) => {
      // Collect `@keyframes` rules to re-insert with theme variables later,
      // since the `@theme` rule itself will be removed.
      if (child.kind === 'rule' && child.selector.startsWith('@keyframes ')) {
        keyframesRules.push(child)
        replaceWith([])
        return WalkAction.Skip
      }

      if (child.kind === 'comment') return
      if (child.kind === 'declaration' && child.property.startsWith('--')) {
        theme.add(child.property, child.value, { isReference, isInline })
        return
      }

      let snippet = toCss([rule(node.selector, [child])])
        .split('\n')
        .map((line, idx, all) => `${idx === 0 || idx >= all.length - 2 ? ' ' : '>'} ${line}`)
        .join('\n')

      throw new Error(
        `\`@theme\` blocks must only contain custom properties or \`@keyframes\`.\n\n${snippet}`,
      )

Review comment (performance):
If you apply the suggestion from above and if you are worried about the `includes` that it needs to check `@media` (first argument of the array), then you can skip it by doing something like this:

```suggestion
    let isReference = themeOptions.includes('reference', 1)
    let isInline = themeOptions.includes('inline', 1)
```
Ultimate performance!

Code after:
      //   }
      // }
      // ```
      else {
        customVariants.push((designSystem) => {
          designSystem.variants.fromAst(name, node.nodes)
        })

        return
      }
    }

    // Drop instances of `@media theme(…)`
    //
    // We support `@import "tailwindcss/theme" theme(reference)` as a way to
    // import an external theme file as a reference, which becomes `@media
    // theme(reference) { … }` when the `@import` is processed.
    if (node.selector.startsWith('@media theme(')) {
      let themeParams = node.selector.slice(13, -1)

      walk(node.nodes, (child) => {
        if (child.kind !== 'rule') {
          throw new Error(
            'Files imported with `@import "…" theme(…)` must only contain `@theme` blocks.',
          )
        }
        if (child.selector === '@theme') {
          child.selector = '@theme ' + themeParams
          return WalkAction.Skip
        }
      })
      replaceWith(node.nodes)
      return WalkAction.Skip
    }

    if (node.selector !== '@theme' && !node.selector.startsWith('@theme ')) return

    let { isReference, isInline } = parseThemeOptions(node.selector)

    // Record all custom properties in the `@theme` declaration
    walk(node.nodes, (child, { replaceWith }) => {
      // Collect `@keyframes` rules to re-insert with theme variables later,
      // since the `@theme` rule itself will be removed.
      if (child.kind === 'rule' && child.selector.startsWith('@keyframes ')) {
        keyframesRules.push(child)
        replaceWith([])
        return WalkAction.Skip
      }

      if (child.kind === 'comment') return
      if (child.kind === 'declaration' && child.property.startsWith('--')) {