I wrote the developer... Rated 5 out of 5 stars

...and he wrote back:

Dear Mark,

Sorry for the late reply - I'm just back from a computer free vacation so there's a lot to catch up with.

It's not that the Red Cats themes are no longer supported - versions for Firefox 14+ are currently in beta stage. You may want to try the latest betas (available by request) or you may wait for a week or two till the final versions are released. Whichever you prefer...

Blue Cat's Graphics

Just thought I'd pass that along.

This review is for a previous version of the add-on (5.5.2).