Most of the CSS files are not located in the UI (admincentral). One can bring additional css easily for server side components via annotation. Unfortunatelly, the CSS for the age editor needs to be explicitly injected in the page editor inner frame.
Make the CSS for the variant selector part of personalization module so the changes can be done independently from the UI.
Espen's resource bundle
This is probably "how it should be done"
Css is parsed and our page-editor.css fails when parsing (all the rules would have to be marked as @external and there are still some parsing errors)