Export newly added theme helpers

Author: Jens Becker

lib/src/extensions/extensions.dart

@@ -1,6 +1,7 @@
 export 'build_context/adaptive_helpers.dart';
 export 'build_context/flashbars.dart';
 export 'build_context/navigation_helpers.dart';
+export 'build_context/theme_helpers.dart';
 
 export 'date_time/helpers.dart';