Draft: Resolve #3149175 "Drupal 9 for 1 x"

Closes #3149175

uikitty.info.yml

 name: uikitty
 type: theme
 description: A building block theme using uikit awesomeness.
-core: 8.x
+# core: 8.x
+core_version_requirement: ^8.8 || ^9
 base theme: classy
 libraries:
   - uikitty/global
@@ -22,4 +23,4 @@ regions:
   offcanvas: 'Off-canvas'
 
 ckeditor_stylesheets:
-  - uikit/css/uikit.almost-flat.min.css
\ No newline at end of file
+  - uikit/css/uikit.almost-flat.min.css