Ensure files with only @theme produce no output when built (#18979)

Closes #18978

Author: thecrypticace

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Use `default` export condition for `@tailwindcss/vite` ([#18948](https://github.com/tailwindlabs/tailwindcss/pull/18948))
 - Re-throw errors from PostCSS nodes ([#18373](https://github.com/tailwindlabs/tailwindcss/pull/18373))
 - Detect classes in markdown inline directives ([#18967](https://github.com/tailwindlabs/tailwindcss/pull/18967))
+- Ensure files with only `@theme` produce no output when built ([#18979](https://github.com/tailwindlabs/tailwindcss/pull/18979))
 
 ## [4.1.13] - 2025-09-03
 

packages/tailwindcss/src/index.test.ts

@@ -5965,4 +5965,18 @@ describe('feature detection', () => {
     expect(compiler.features & Features.AtImport).toBeTruthy()
     expect(compiler.features & Features.Utilities).toBeFalsy()
   })
+
+  test('using `@theme`', async () => {
+    let compiler = await compile(css`
+      @theme {
+        --tracking-narrower: -0.02em;
+      }
+    `)
+
+    // We see @theme
+    expect(compiler.features & Features.AtTheme).toBeTruthy()
+
+    // And this produces no output because no other CSS is present
+    expect(compiler.build([])).toEqual('')
+  })
 })

packages/tailwindcss/src/index.ts

@@ -133,6 +133,9 @@ export const enum Features {
 
   // `@variant` was used
   Variants = 1 << 5,
+
+  // `@theme` was used
+  AtTheme = 1 << 6,
 }
 
 async function parseCss(
@@ -540,6 +543,8 @@ async function parseCss(
     if (node.name === '@theme') {
       let [themeOptions, themePrefix] = parseThemeOptions(node.params)
 
+      features |= Features.AtTheme
+
       if (context.reference) {
         themeOptions |= ThemeOptions.REFERENCE
       }