Add --border-color to divide theme keys (#18704)

## Summary

In Tailwind 3 the border colors were able to be used with `divide`
utilities. I made it so that's true for Tailwind 4.

## Test plan

Just used `pnpm run tdd` and making it fails then making sure it passes.

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add option to disable url rewriting in `@tailwindcss/postcss` ([#18321](https://github.com/tailwindlabs/tailwindcss/pull/18321))
 - Fix false-positive migrations in `addEventListener` and JavaScript variable names ([#18718](https://github.com/tailwindlabs/tailwindcss/pull/18718))
 - Fix Standalone CLI when run via symlink on Windows ([#18723](https://github.com/tailwindlabs/tailwindcss/pull/18723))
+- Read from `--border-color-*` theme keys in `divide-*` utilities for backwards compatibility ([#18704](https://github.com/tailwindlabs/tailwindcss/pull/18704/))
 
 ## [4.1.11] - 2025-06-26
 

packages/tailwindcss/src/utilities.test.ts

@@ -8843,6 +8843,7 @@ test('divide-color', async () => {
       css`
         @theme {
           --color-red-500: #ef4444;
+          --border-color-best-blue: #6495ED;
         }
         @tailwind utilities;
       `,
@@ -8854,6 +8855,7 @@ test('divide-color', async () => {
         'divide-red-500/2.75',
         'divide-red-500/[0.5]',
         'divide-red-500/[50%]',
+        'divide-best-blue',
         'divide-current',
         'divide-current/50',
         'divide-current/[0.5]',
@@ -8869,6 +8871,7 @@ test('divide-color', async () => {
   ).toMatchInlineSnapshot(`
     ":root, :host {
       --color-red-500: #ef4444;
+      --border-color-best-blue: #6495ed;
     }
 
     :where(.divide-\\[\\#0088cc\\] > :not(:last-child)) {
@@ -8879,6 +8882,10 @@ test('divide-color', async () => {
       border-color: oklab(59.9824% -.067 -.124 / .5);
     }
 
+    :where(.divide-best-blue > :not(:last-child)) {
+      border-color: var(--border-color-best-blue);
+    }
+
     :where(.divide-current > :not(:last-child)), :where(.divide-current\\/50 > :not(:last-child)) {
       border-color: currentColor;
     }

packages/tailwindcss/src/utilities.ts

@@ -2007,7 +2007,7 @@ export function createUtilities(theme: Theme) {
   })
 
   colorUtility('divide', {
-    themeKeys: ['--divide-color', '--color'],
+    themeKeys: ['--divide-color', '--border-color', '--color'],
     handle: (value) => [
       styleRule(':where(& > :not(:last-child))', [
         decl('--tw-sort', 'divide-color'),