diff --git a/CHANGELOG.md b/CHANGELOG.md
index 21c1bef88..3404eea0e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Allow negative utility names in `@utilty` ([#15573](https://github.com/tailwindlabs/tailwindcss/pull/15573))
 - Remove all `@keyframes` contributed by JavaScript plugins when using `@reference` imports ([#15581](https://github.com/tailwindlabs/tailwindcss/pull/15581))
 - _Upgrade (experimental)_: Do not extract class names from functions (e.g. `shadow` in `filter: 'drop-shadow(…)'`) ([#15566](https://github.com/tailwindlabs/tailwindcss/pull/15566))
+- _Upgrade (experimental)_: Migrate `theme(spacing.2)` to `--spacing(2)` ([#15579](https://github.com/tailwindlabs/tailwindcss/pull/15579))
+- _Upgrade (experimental)_: Migrate `theme(…)` to `--theme(…)` ([#15579](https://github.com/tailwindlabs/tailwindcss/pull/15579))
 
 ### Changed
 
diff --git a/integrations/upgrade/js-config.test.ts b/integrations/upgrade/js-config.test.ts
index e9ad772f6..56c77189c 100644
--- a/integrations/upgrade/js-config.test.ts
+++ b/integrations/upgrade/js-config.test.ts
@@ -1296,10 +1296,10 @@ describe('border compatibility', () => {
         "
         --- src/index.html ---
         <div
-          class="[width:calc(var(--spacing)*2)]
-            [width:calc(var(--spacing)*4.5)]
+          class="[width:--spacing(2)]
+            [width:--spacing(4.5)]
             [width:var(--spacing-5_5)]
-            [width:calc(var(--spacing)*13)]
+            [width:--spacing(13)]
             [width:var(--spacing-100)]
             [width:var(--spacing-miami)]"
         ></div>
@@ -1332,10 +1332,10 @@ describe('border compatibility', () => {
         }
 
         .container {
-          width: calc(var(--spacing) * 2);
-          width: calc(var(--spacing) * 4.5);
+          width: --spacing(2);
+          width: --spacing(4.5);
           width: var(--spacing-5_5);
-          width: calc(var(--spacing) * 13);
+          width: --spacing(13);
           width: var(--spacing-100);
           width: var(--spacing-miami);
         }
@@ -1515,7 +1515,7 @@ describe('border compatibility', () => {
         @utility container {
           margin-inline: auto;
           padding-inline: 2rem;
-          @media (width >= theme(--breakpoint-sm)) {
+          @media (width >= --theme(--breakpoint-sm)) {
             max-width: none;
           }
           @media (width >= 48rem) {
diff --git a/packages/@tailwindcss-upgrade/src/codemods/migrate-theme-to-var.test.ts b/packages/@tailwindcss-upgrade/src/codemods/migrate-theme-to-var.test.ts
index ce3940bc9..f0b4844ad 100644
--- a/packages/@tailwindcss-upgrade/src/codemods/migrate-theme-to-var.test.ts
+++ b/packages/@tailwindcss-upgrade/src/codemods/migrate-theme-to-var.test.ts
@@ -35,11 +35,11 @@ it('should migrate `theme(…)` to `var(…)`', async () => {
       }
     `),
   ).toMatchInlineSnapshot(`
-    "@media theme(--breakpoint-sm) {
+    "@media --theme(--breakpoint-sm) {
       .foo {
         background-color: var(--color-red-900);
-        color: theme(--color-red-900 / 75%);
-        border-color: theme(--color-red-200 / 75%);
+        color: --theme(--color-red-900 / 75%);
+        border-color: --theme(--color-red-200 / 75%);
       }
     }"
   `)
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.test.ts b/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.test.ts
index d9e714e9e..59b1523b7 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.test.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.test.ts
@@ -36,6 +36,10 @@ test.each([
   ['supports-[--test]:flex', 'supports-(--test):flex'],
   ['supports-[_--test]:flex', 'supports-[--test]:flex'],
 
+  // Custom CSS functions that look like variables should not be converted
+  ['w-[--spacing(5)]', 'w-[--spacing(5)]'],
+  ['bg-[--theme(--color-red-500)]', 'bg-[--theme(--color-red-500)]'],
+
   // Some properties never had var() injection in v3.
   ['[scroll-timeline-name:--myTimeline]', '[scroll-timeline-name:--myTimeline]'],
   ['[timeline-scope:--myScope]', '[timeline-scope:--myScope]'],
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.ts b/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.ts
index 33995c04a..643d000c6 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.ts
@@ -74,13 +74,14 @@ export function automaticVarInjection(
 
 function injectVar(value: string): { value: string; didChange: boolean } {
   let didChange = false
-  if (value.startsWith('--')) {
+  if (value.startsWith('--') && !value.includes('(')) {
     value = `var(${value})`
     didChange = true
   } else if (value.startsWith(' --')) {
     value = value.slice(1)
     didChange = true
   }
+
   return { value, didChange }
 }
 
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.test.ts b/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.test.ts
index f5f2a41db..2ff074010 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.test.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.test.ts
@@ -9,9 +9,9 @@ test.each([
   ['[color:red]', '[color:red]'],
 
   // Handle special cases around `.1` in the `theme(…)`
-  ['[--value:theme(spacing.1)]', '[--value:calc(var(--spacing)*1)]'],
+  ['[--value:theme(spacing.1)]', '[--value:--spacing(1)]'],
   ['[--value:theme(fontSize.xs.1.lineHeight)]', '[--value:var(--text-xs--line-height)]'],
-  ['[--value:theme(spacing[1.25])]', '[--value:calc(var(--spacing)*1.25)]'],
+  ['[--value:theme(spacing[1.25])]', '[--value:--spacing(1.25)]'],
 
   // Should not convert invalid spacing values to calc
   ['[--value:theme(spacing[1.1])]', '[--value:theme(spacing[1.1])]'],
@@ -20,7 +20,7 @@ test.each([
   ['[color:theme(colors.red.500)]', '[color:var(--color-red-500)]'], // Arbitrary property
   ['[color:theme(colors.red.500)]/50', '[color:var(--color-red-500)]/50'], // Arbitrary property + modifier
   ['bg-[theme(colors.red.500)]', 'bg-(--color-red-500)'], // Arbitrary value
-  ['bg-[size:theme(spacing.4)]', 'bg-[size:calc(var(--spacing)*4)]'], // Arbitrary value + data type hint
+  ['bg-[size:theme(spacing.4)]', 'bg-[size:--spacing(4)]'], // Arbitrary value + data type hint
 
   // Convert to `var(…)` if we can resolve the path, but keep fallback values
   ['bg-[theme(colors.red.500,red)]', 'bg-(--color-red-500,red)'],
@@ -47,11 +47,11 @@ test.each([
   //   to a candidate modifier _if_ all `theme(…)` calls use the same modifier.
   [
     '[color:theme(colors.red.500/50,theme(colors.blue.500/50))]',
-    '[color:theme(--color-red-500/50,theme(--color-blue-500/50))]',
+    '[color:--theme(--color-red-500/50,--theme(--color-blue-500/50))]',
   ],
   [
     '[color:theme(colors.red.500/50,theme(colors.blue.500/50))]/50',
-    '[color:theme(--color-red-500/50,theme(--color-blue-500/50))]/50',
+    '[color:--theme(--color-red-500/50,--theme(--color-blue-500/50))]/50',
   ],
 
   // Convert the `theme(…)`, but try to move the inline modifier (e.g. `50%`),
@@ -75,22 +75,22 @@ test.each([
   ['bg-[theme(colors.red.500/12.34%)]', 'bg-(--color-red-500)/[12.34%]'],
 
   // Arbitrary property that already contains a modifier
-  ['[color:theme(colors.red.500/50%)]/50', '[color:theme(--color-red-500/50%)]/50'],
+  ['[color:theme(colors.red.500/50%)]/50', '[color:--theme(--color-red-500/50%)]/50'],
 
   // Values that don't contain only `theme(…)` calls should not be converted to
   // use a modifier since the color is not the whole value.
   [
     'shadow-[shadow:inset_0px_1px_theme(colors.white/15%)]',
-    'shadow-[shadow:inset_0px_1px_theme(--color-white/15%)]',
+    'shadow-[shadow:inset_0px_1px_--theme(--color-white/15%)]',
   ],
 
   // Arbitrary value, where the candidate already contains a modifier
   // This should still migrate the `theme(…)` syntax to the modern syntax.
-  ['bg-[theme(colors.red.500/50%)]/50', 'bg-[theme(--color-red-500/50%)]/50'],
+  ['bg-[theme(colors.red.500/50%)]/50', 'bg-[--theme(--color-red-500/50%)]/50'],
 
   // Variants, we can't use `var(…)` especially inside of `@media(…)`. We can
   // still upgrade the `theme(…)` to the modern syntax.
-  ['max-[theme(screens.lg)]:flex', 'max-[theme(--breakpoint-lg)]:flex'],
+  ['max-[theme(screens.lg)]:flex', 'max-[--theme(--breakpoint-lg)]:flex'],
   // There are no variables for `--spacing` multiples, so we can't convert this
   ['max-[theme(spacing.4)]:flex', 'max-[theme(spacing.4)]:flex'],
 
@@ -100,10 +100,7 @@ test.each([
 
   // `theme(…)` calls in another CSS function is replaced correctly.
   // Additionally we remove unnecessary whitespace.
-  [
-    'grid-cols-[min(50%_,_theme(spacing.80))_auto]',
-    'grid-cols-[min(50%,calc(var(--spacing)*80))_auto]',
-  ],
+  ['grid-cols-[min(50%_,_theme(spacing.80))_auto]', 'grid-cols-[min(50%,--spacing(80))_auto]'],
 
   // `theme(…)` calls valid in v3, but not in v4 should still be converted.
   ['[--foo:theme(transitionDuration.500)]', '[--foo:theme(transitionDuration.500)]'],
@@ -152,7 +149,7 @@ test('extended space scale converts to var or calc', async () => {
     },
   )
   expect(themeToVar(designSystem, {}, '[--value:theme(spacing.1)]')).toEqual(
-    '[--value:calc(var(--spacing)*1)]',
+    '[--value:--spacing(1)]',
   )
   expect(themeToVar(designSystem, {}, '[--value:theme(spacing.2)]')).toEqual(
     '[--value:var(--spacing-2)]',
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.ts b/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.ts
index b83e5add1..128b5e8b8 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/theme-to-var.ts
@@ -148,9 +148,7 @@ export function createConverter(designSystem: DesignSystem, { prettyPrint = fals
       let parts = segment(path, '/').map((part) => part.trim())
 
       // Multiple `/` separators, which makes this an invalid path
-      if (parts.length > 2) {
-        return null
-      }
+      if (parts.length > 2) return null
 
       // The path contains a `/`, which means that there is a modifier such as
       // `theme(colors.red.500/50%)`.
@@ -212,7 +210,7 @@ export function createConverter(designSystem: DesignSystem, { prettyPrint = fals
       let multiplier = keyPath[1]
       if (!isValidSpacingMultiplier(multiplier)) return null
 
-      return 'calc(var(--spacing) * ' + multiplier + ')'
+      return `--spacing(${multiplier})`
     }
 
     return null
@@ -227,7 +225,9 @@ export function createConverter(designSystem: DesignSystem, { prettyPrint = fals
 
     let modifier =
       parts.length > 0 ? (prettyPrint ? ` / ${parts.join(' / ')}` : `/${parts.join('/')}`) : ''
-    return fallback ? `theme(${variable}${modifier}, ${fallback})` : `theme(${variable}${modifier})`
+    return fallback
+      ? `--theme(${variable}${modifier}, ${fallback})`
+      : `--theme(${variable}${modifier})`
   }
 
   return convert
diff --git a/packages/tailwindcss/src/compat/container.ts b/packages/tailwindcss/src/compat/container.ts
index 99aea1e42..3cf72c4db 100644
--- a/packages/tailwindcss/src/compat/container.ts
+++ b/packages/tailwindcss/src/compat/container.ts
@@ -60,7 +60,7 @@ export function buildCustomContainerUtilityRules(
       let [key] = breakpoints[0]
       // Unset all default breakpoints
       rules.push(
-        atRule('@media', `(width >= theme(--breakpoint-${key}))`, [decl('max-width', 'none')]),
+        atRule('@media', `(width >= --theme(--breakpoint-${key}))`, [decl('max-width', 'none')]),
       )
     }