Wrap adaptV4Theme with createV5Theme to support extra vars

Signed-off-by: Ilya Savich <isavich@box.com>

.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

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));
 }