diff --git a/.changeset/hungry-days-remain.md b/.changeset/hungry-days-remain.md
new file mode 100644
index 0000000000..b0c3642a35
--- /dev/null
+++ b/.changeset/hungry-days-remain.md
@@ -0,0 +1,5 @@
+---
+'@backstage/theme': patch
+---
+
+Fixed missing extra variables like `applyDarkStyles` in Mui V5 theme after calling `createUnifiedThemeFromV4` function
diff --git a/packages/theme/src/unified/UnifiedTheme.tsx b/packages/theme/src/unified/UnifiedTheme.tsx
index aeae9694ee..d6612d0a1d 100644
--- a/packages/theme/src/unified/UnifiedTheme.tsx
+++ b/packages/theme/src/unified/UnifiedTheme.tsx
@@ -93,5 +93,5 @@ export function createUnifiedThemeFromV4(
 ): UnifiedTheme {
   const v5Theme = adaptV4Theme(options as DeprecatedThemeOptions);
   const v4Theme = createTheme(options);
-  return new UnifiedThemeHolder(v4Theme, v5Theme);
+  return new UnifiedThemeHolder(v4Theme, createV5Theme(v5Theme));
 }