Don’t unconditionally convert config keys to kebab-case

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Skip comments in Ruby files when checking for class names ([#19243](https://github.com/tailwindlabs/tailwindcss/pull/19243))
 - Skip over arbitrary property utilities with a top-level `!` in the value ([#19243](https://github.com/tailwindlabs/tailwindcss/pull/19243))
 - Support environment API in `@tailwindcss/vite` ([#18970](https://github.com/tailwindlabs/tailwindcss/pull/18970))
+- Preserve case of theme keys from JS configs and plugins ([#19337](https://github.com/tailwindlabs/tailwindcss/pull/19337))
 - Upgrade: Handle `future` and `experimental` config keys ([#19344](https://github.com/tailwindlabs/tailwindcss/pull/19344))
 
 ### Added

integrations/upgrade/js-config.test.ts

@@ -167,9 +167,9 @@ test(
       "
       --- src/index.html ---
       <div
-        class="tracking-super-wide leading-super-loose"
+        class="tracking-superWide leading-superLoose"
       ></div>
-      <div class="text-super-red/super-opaque leading-super-loose"></div>
+      <div class="text-superRed/superOpaque leading-superLoose"></div>
 
       --- src/input.css ---
       @import 'tailwindcss';
@@ -187,12 +187,12 @@ test(
         --color-red-500: #ef4444;
         --color-red-600: #dc2626;
 
-        --color-super-red: #ff0000;
+        --color-superRed: #ff0000;
         --color-steel: rgb(70 130 180);
         --color-smoke: rgba(245, 245, 245, var(--smoke-alpha, 1));
 
         --opacity-*: initial;
-        --opacity-super-opaque: 95%;
+        --opacity-superOpaque: 95%;
 
         --text-*: initial;
         --text-xs: 0.75rem;
@@ -265,9 +265,9 @@ test(
         --animate-spin-clockwise: spin-clockwise 1s linear infinite;
         --animate-spin-counterclockwise: spin-counterclockwise 1s linear infinite;
 
-        --tracking-super-wide: 0.25em;
+        --tracking-superWide: 0.25em;
 
-        --leading-super-loose: 3;
+        --leading-superLoose: 3;
 
         @keyframes spin-clockwise {
           0% {

packages/tailwindcss/src/compat/apply-config-to-theme.test.ts

@@ -118,8 +118,8 @@ test('config values can be merged into the theme', () => {
   ])
   expect(theme.resolve('2xl', ['--text'])).toEqual('2rem')
   expect(theme.resolveWith('2xl', ['--text'], ['--line-height'])).toEqual(['2rem', {}])
-  expect(theme.resolve('super-wide', ['--tracking'])).toEqual('0.25em')
-  expect(theme.resolve('super-loose', ['--leading'])).toEqual('3')
+  expect(theme.resolve('superWide', ['--tracking'])).toEqual('0.25em')
+  expect(theme.resolve('superLoose', ['--leading'])).toEqual('3')
   expect(theme.resolve('1/2', ['--width'])).toEqual('60%')
   expect(theme.resolve('0.5', ['--width'])).toEqual('60%')
   expect(theme.resolve('100%', ['--width'])).toEqual('100%')

packages/tailwindcss/src/compat/apply-config-to-theme.ts

@@ -185,11 +185,27 @@ export function keyPathToCssProperty(path: string[]) {
       .map((path, idx, all) => (path === '1' && idx !== all.length - 1 ? '' : path))
 
       // Resolve the key path to a CSS variable segment
-      .map((part) =>
-        part
-          .replaceAll('.', '_')
-          .replace(/([a-z])([A-Z])/g, (_, a, b) => `${a}-${b.toLowerCase()}`),
-      )
+      .map((part, idx) => {
+        part = part.replaceAll('.', '_')
+
+        let shouldConvert =
+          // The first "namespace" part should be converted to kebab-case
+          // This converts things like backgroundColor to `background-color`
+          idx === 0 ||
+          // Any tuple nested key should be converted to kebab-case
+          // These are identified with a leading `-`
+          // e.g. `fontSize.xs.1.lineHeight` -> `font-size-xs--line-height`
+          part.startsWith('-') ||
+          // `lineHeight` is a bit of a special case in which it does not
+          // always begin with a leading `-` even when as a nested tuple key
+          part === 'lineHeight'
+
+        if (shouldConvert) {
+          part = part.replace(/([a-z])([A-Z])/g, (_, a, b) => `${a}-${b.toLowerCase()}`)
+        }
+
+        return part
+      })
 
       // Remove the `DEFAULT` key at the end of a path
       // We're reading from CSS anyway so it'll be a string

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

@@ -1719,3 +1719,55 @@ test('The theme() function does not try indexing into strings', async () => {
     "
   `)
 })
+
+test('camel case keys are preserved', async () => {
+  let compiler = await compile(
+    css`
+      @tailwind utilities;
+      @theme {
+        --color-blue-green: slate;
+      }
+      @config "./plugin.js";
+    `,
+    {
+      loadModule: async () => {
+        return {
+          base: '/',
+          path: '',
+          module: {
+            theme: {
+              extend: {
+                backgroundColor: {
+                  lightGreen: '#c0ffee',
+                },
+              },
+            },
+          },
+        }
+      },
+    },
+  )
+
+  expect(
+    compiler.build([
+      // From CSS
+      'bg-blue-green', // should be output
+      'bg-blueGreen', // should not
+
+      // From JS config
+      'bg-light-green', // should not be output
+      'bg-lightGreen', // should be
+    ]),
+  ).toMatchInlineSnapshot(`
+    ".bg-blue-green {
+      background-color: var(--color-blue-green);
+    }
+    .bg-lightGreen {
+      background-color: #c0ffee;
+    }
+    :root, :host {
+      --color-blue-green: slate;
+    }
+    "
+  `)
+})