add import alias for css

src/Umbraco.Web.UI.Client/src/app.ts

@@ -1,4 +1,4 @@
-import './core/css/custom-properties.css';
+import '@umbraco-cms/css/custom-properties.css';
 import 'router-slot';
 
 // TODO: remove these imports when they are part of UUI

src/Umbraco.Web.UI.Client/tsconfig.json

@@ -30,6 +30,7 @@
       "@umbraco-cms/extensions-api": ["src/core/extensions-api"],
       "@umbraco-cms/observable-api": ["src/core/observable-api"],
       "@umbraco-cms/utils": ["src/core/utils"],
+      "@umbraco-cms/css/*": ["src/core/css/*"],
     },
   },
   "include": [