If they are not needed anymore, we can remove all code related to the old themes (= lots of CSS).

Remove the old version from the dropdown, and remove the "(new)" suffix on the new version.

Some themes ("Arctic", "Antarctic", and "Oil") were not converted because they're not used by any live content, these are also removed. In the unlikely event that someone was about to use one of these, we can still create a new version for them.