More thanks to MT7523 Rated 5 out of 5 stars

I now have my Red Cats (Blue Flavour) back thanks to the link posted below - phew!!!

Initially the recompressed file wasn't installing and I got a message saying it was corrupt. I found by googling that to recompress the uncompressed file to make a .jar file you don't recompress the folder but select all items within the folder and compress them. Then it's the resulting Archive.zip file that needs to be renamed red_cats_blue_flavor-5.5.2-fx.jar .

This is using a Mac. Hope it helps anyone else still trying.

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