## Summary
When a CSS theme key defined via `@theme` (or a JS config's `theme`
object) shares a dash-separated prefix with a sibling key — e.g.
`--color-foo` and `--color-foo-bar` — calling `theme('colors.foo')` from
inside a JS plugin (`addUtilities`, `addComponents`, etc.) does not
resolve to the `foo` value. Instead it returns an internal
disambiguation object shaped like `{ DEFAULT: 'red', bar: 'blue',
__CSS_VALUES__: {...} }`, because there's no way to tell from CSS custom
property names alone whether `foo-bar` is a sibling key or a nested
sub-key of `foo`.
This same ambiguity was already fixed for the CSS-embedded `theme()`
function in #19097 (which unwraps to the `DEFAULT` key when present),
and the changelog entry for that PR states it fixes this "in JS configs
**and plugins**" — but the fix only touched `apply-compat-hooks.ts`'s
`resolveThemeValue`, not `createThemeFn`'s `theme` function that's
exposed directly to plugins in `plugin-functions.ts`. This PR closes
that gap by applying the same DEFAULT-unwrapping there.
Without this fix, passing the raw object into `addUtilities` (a very
natural thing to do, since a plugin author expects a string) produces
broken CSS — the reserved `DEFAULT` key gets mangled into a garbage
property name (`-d-e-f-a-u-l-t`) by the kebab-case conversion, and the
internal `__CSS_VALUES__` bookkeeping leaks into the generated
stylesheet.
### Minimal reproduction
```js
// tailwind.config.js (registered via @config, or any @plugin-registered plugin)
const plugin = require('tailwindcss/plugin')
module.exports = {
plugins: [
plugin(function ({ addUtilities, theme }) {
addUtilities({
'.example-foo': { color: theme('colors.foo') },
})
}),
],
}
```
```css
@import "tailwindcss";
@config "./tailwind.config.js";
@theme {
--color-foo: red;
--color-foo-bar: blue;
}
```
**Before:**
```css
.example-foo color {
-d-e-f-a-u-l-t: red;
bar: blue;
}
.example-foo color __CSS_VALUES__ {
-d-e-f-a-u-l-t: 0;
bar: 0;
}
```
**After:**
```css
.example-foo {
color: red;
}
```
## Test plan
- Added a regression test in
`packages/tailwindcss/src/compat/plugin-api.test.ts` ("theme() resolves
the DEFAULT value when a bare CSS theme key shares a prefix with a
sibling key")
- Verified via `pnpm --filter tailwindcss exec vitest run` that this
test fails with the exact broken output shown above when the fix is
reverted, and passes once it's applied
- Ran the full `tailwindcss` package test suite (`pnpm --filter
tailwindcss exec vitest run`) — 4684 tests passing, no regressions
- Verified formatting on the changed files with `npx prettier --check`
---------
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>
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