Hi

I believe that if you are using one of the new colorways, it will stay installed in Firefox when the offer ends, but you will not be able to reinstall it or change it to another (although plenty of themes are available at https://addons.mozilla.org/en-US/firefox/themes/).

Where are you getting these themes from? If they aren't from Mozilla/Firefox then firefox can't do much about it. That would mean you need to contact the site your getting those themes from for more help.