Fix resolving colors via theme(…) in compat mode with nested objects (#19097)

This PR fixes an issue when loading (nested) colors from a config file
and later referencing it via the `theme(…)` function in CSS.

Given a config like this:

```js
module.exports = {
  theme: {
    colors: {
      foo: 'var(--foo-foo)',
      'foo-bar': 'var(--foo-foo-bar)',
    },
  },
}
```

We internally map this into the design system. The issue here is that
the `foo` and `foo-bar` are overlapping and it behaves more like this:

```js
{
  foo: {
    DEFAULT: 'var(--foo-foo)',
    bar: 'var(--foo-foo-bar)'
  },
}
```

So while we can easily resolve `colors.foo-bar`, the `colors.foo` would
result in the object with a `DEFAULT` key. This PR solves that by using
the `DEFAULT` key if we end up with an object that has it.

If you end up resolving an object (`theme(colors)`) then the behavior is
unchanged.


## Test plan

1. Added a test based on the config in the issue (which failed before
this fix).
2. Also simplified the test case after identifying the problem (with the
`DEFAULT` key).

Fixes: #19091

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Fix Safari devtools rendering issue due to `color-mix` fallback ([#19069](https://github.com/tailwindlabs/tailwindcss/pull/19069))
 - Suppress Lightning CSS warnings about `:deep`, `:slotted` and `:global` ([#19094](https://github.com/tailwindlabs/tailwindcss/pull/19094))
+- Fix resolving theme keys when starting with the name of another theme key in JS configs and plugins ([#19097](https://github.com/tailwindlabs/tailwindcss/pull/19097))
 
 ## [4.1.14] - 2025-10-01
 

packages/@tailwindcss-upgrade/src/codemods/css/migrate-media-screen.ts

@@ -16,7 +16,7 @@ export function migrateMediaScreen({
     if (!designSystem || !userConfig) return
 
     let { resolvedConfig } = resolveConfig(designSystem, [
-      { base: '', config: userConfig, reference: false },
+      { base: '', config: userConfig, reference: false, src: undefined },
     ])
     let screens = resolvedConfig?.theme?.screens || {}
 

packages/tailwindcss/src/compat/apply-compat-hooks.ts

@@ -309,15 +309,29 @@ function upgradeToFullPluginSupport({
 
     let resolvedValue = sharedPluginApi.theme(path, undefined)
 
+    // When a tuple is returned, return the first element
     if (Array.isArray(resolvedValue) && resolvedValue.length === 2) {
-      // When a tuple is returned, return the first element
       return resolvedValue[0]
-    } else if (Array.isArray(resolvedValue)) {
-      // Arrays get serialized into a comma-separated lists
+    }
+
+    // Arrays get serialized into a comma-separated lists
+    else if (Array.isArray(resolvedValue)) {
       return resolvedValue.join(', ')
-    } else if (typeof resolvedValue === 'string') {
-      // Otherwise only allow string values here, objects (and namespace maps)
-      // are treated as non-resolved values for the CSS `theme()` function.
+    }
+
+    // If we're dealing with an object that has the `DEFAULT` key, return the
+    // default value
+    else if (
+      typeof resolvedValue === 'object' &&
+      resolvedValue !== null &&
+      'DEFAULT' in resolvedValue
+    ) {
+      return resolvedValue.DEFAULT
+    }
+
+    // Otherwise only allow string values here, objects (and namespace maps)
+    // are treated as non-resolved values for the CSS `theme()` function.
+    else if (typeof resolvedValue === 'string') {
       return resolvedValue
     }
   }

packages/tailwindcss/src/compat/config.test.ts

@@ -12,7 +12,7 @@ test('Config files can add content', async () => {
   `
 
   let compiler = await compile(input, {
-    loadModule: async () => ({ module: { content: ['./file.txt'] }, base: '/root' }),
+    loadModule: async () => ({ module: { content: ['./file.txt'] }, base: '/root', path: '' }),
   })
 
   expect(compiler.sources).toEqual([{ base: '/root', pattern: './file.txt', negated: false }])
@@ -25,7 +25,7 @@ test('Config files can change dark mode (media)', async () => {
   `
 
   let compiler = await compile(input, {
-    loadModule: async () => ({ module: { darkMode: 'media' }, base: '/root' }),
+    loadModule: async () => ({ module: { darkMode: 'media' }, base: '/root', path: '' }),
   })
 
   expect(compiler.build(['dark:underline'])).toMatchInlineSnapshot(`
@@ -45,7 +45,7 @@ test('Config files can change dark mode (selector)', async () => {
   `
 
   let compiler = await compile(input, {
-    loadModule: async () => ({ module: { darkMode: 'selector' }, base: '/root' }),
+    loadModule: async () => ({ module: { darkMode: 'selector' }, base: '/root', path: '' }),
   })
 
   expect(compiler.build(['dark:underline'])).toMatchInlineSnapshot(`
@@ -68,6 +68,7 @@ test('Config files can change dark mode (variant)', async () => {
     loadModule: async () => ({
       module: { darkMode: ['variant', '&:where(:not(.light))'] },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -101,6 +102,7 @@ test('Config files can add plugins', async () => {
         ],
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -128,6 +130,7 @@ test('Plugins loaded from config files can contribute to the config', async () =
         ],
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -157,6 +160,7 @@ test('Config file presets can contribute to the config', async () => {
         ],
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -198,6 +202,7 @@ test('Config files can affect the theme', async () => {
         ],
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -212,6 +217,48 @@ test('Config files can affect the theme', async () => {
   `)
 })
 
+// https://github.com/tailwindlabs/tailwindcss/issues/19091
+test('Accessing a default color if a sub-color exists via CSS should work as expected', async () => {
+  let input = css`
+    @tailwind utilities;
+    @config "./config.js";
+
+    .example {
+      color: theme('colors.foo-bar');
+      border-color: theme('colors.foo');
+    }
+  `
+
+  let compiler = await compile(input, {
+    loadModule: async () => ({
+      module: {
+        theme: {
+          // Internally this object gets converted to something like:
+          // ```
+          // {
+          //   foo: { DEFAULT: 'var(--foo-foo)', bar: 'var(--foo-foo-bar)' },
+          // }
+          // ```
+          colors: {
+            foo: 'var(--foo-foo)',
+            'foo-bar': 'var(--foo-foo-bar)',
+          },
+        },
+      },
+      base: '/root',
+      path: '',
+    }),
+  })
+
+  expect(compiler.build([])).toMatchInlineSnapshot(`
+    ".example {
+      color: var(--foo-foo-bar);
+      border-color: var(--foo-foo);
+    }
+    "
+  `)
+})
+
 test('Variants in CSS overwrite variants from plugins', async () => {
   let input = css`
     @tailwind utilities;
@@ -231,6 +278,7 @@ test('Variants in CSS overwrite variants from plugins', async () => {
         ],
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -317,6 +365,7 @@ describe('theme callbacks', () => {
           ],
         } satisfies Config,
         base: '/root',
+        path: '',
       }),
     })
 
@@ -391,6 +440,7 @@ describe('theme overrides order', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -442,6 +492,7 @@ describe('theme overrides order', () => {
               },
             } satisfies Config,
             base: '/root',
+            path: '',
           }
         } else {
           return {
@@ -460,6 +511,7 @@ describe('theme overrides order', () => {
               )
             }),
             base: '/root',
+            path: '',
           }
         }
       },
@@ -562,6 +614,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -596,6 +649,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -631,6 +685,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -669,6 +724,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -708,6 +764,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -745,6 +802,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -779,6 +837,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -806,6 +865,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -840,6 +900,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -875,6 +936,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -913,6 +975,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -952,6 +1015,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -989,6 +1053,7 @@ describe('default font family compatibility', () => {
           },
         },
         base: '/root',
+        path: '',
       }),
     })
 
@@ -1021,6 +1086,7 @@ test('creates variants for `data`, `supports`, and `aria` theme options at the s
         },
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -1113,6 +1179,7 @@ test('merges css breakpoints with js config screens', async () => {
         },
       },
       base: '/root',
+      path: '',
     }),
   })
 
@@ -1596,6 +1663,7 @@ test('handles setting theme keys to null', async () => {
             },
           },
           base: '/root',
+          path: '',
         }
       },
     },