Support CSS theme() functions inside @custom-media rules

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Support CSS `theme()` functions inside other `@custom-media`, `@container`, and `@supports` rules ([#14358])(https://github.com/tailwindlabs/tailwindcss/pull/14358)
+
 ### Fixed
 
 - Ensure there is always CLI feedback on save even when no new classes were found ([#14351](https://github.com/tailwindlabs/tailwindcss/pull/14351))

packages/tailwindcss/src/css-functions.test.ts

@@ -619,6 +619,82 @@ describe('theme function', () => {
       `)
     })
   })
+
+  test('@custom-media --my-media (min-width: theme(breakpoint.md))', async () => {
+    expect(
+      await compileCss(css`
+        @theme {
+          --breakpoint-md: 48rem;
+        }
+        @custom-media --my-media (min-width: theme(breakpoint.md));
+        @media (--my-media) {
+          .red {
+            color: red;
+          }
+        }
+      `),
+    ).toMatchInlineSnapshot(`
+      ":root {
+        --breakpoint-md: 48rem;
+      }
+
+      @media (width >= 48rem) {
+        .red {
+          color: red;
+        }
+      }"
+    `)
+  })
+
+  test('@container (width > theme(breakpoint.md))', async () => {
+    expect(
+      await compileCss(css`
+        @theme {
+          --breakpoint-md: 48rem;
+        }
+        @container (width > theme(breakpoint.md)) {
+          .red {
+            color: red;
+          }
+        }
+      `),
+    ).toMatchInlineSnapshot(`
+      ":root {
+        --breakpoint-md: 48rem;
+      }
+
+      @container (width > 48rem) {
+        .red {
+          color: red;
+        }
+      }"
+    `)
+  })
+
+  test('@supports (text-stroke: theme(--font-size-xs))', async () => {
+    expect(
+      await compileCss(css`
+        @theme {
+          --font-size-xs: 0.75rem;
+        }
+        @supports (text-stroke: theme(--font-size-xs)) {
+          .red {
+            color: red;
+          }
+        }
+      `),
+    ).toMatchInlineSnapshot(`
+      ":root {
+        --font-size-xs: .75rem;
+      }
+
+      @supports (text-stroke: 0.75rem) {
+        .red {
+          color: red;
+        }
+      }"
+    `)
+  })
 })
 
 describe('in plugins', () => {

packages/tailwindcss/src/css-functions.ts

@@ -13,11 +13,14 @@ export function substituteFunctions(ast: AstNode[], pluginApi: PluginAPI) {
       return
     }
 
-    // Find @media rules
+    // Find at-rules rules
     if (node.kind === 'rule') {
       if (
         node.selector[0] === '@' &&
-        node.selector.startsWith('@media ') &&
+        (node.selector.startsWith('@media ') ||
+          node.selector.startsWith('@custom-media ') ||
+          node.selector.startsWith('@container ') ||
+          node.selector.startsWith('@supports ')) &&
         node.selector.includes(THEME_FUNCTION_INVOCATION)
       ) {
         node.selector = substituteFunctionsInValue(node.selector, pluginApi)