Rated 5 out of 5 stars

Incredibly useful. Can't believe this isn't part of Stylish already.

Is there a way to limit the number of backups kept, besides turning them off entirely?

This review is for a previous version of the add-on (0.1.6.1-signed). 

Yes, there is a way...

If you're familiar with 'about:config', you can go there and set the preference 'extensions.stylishsync.bakage'. The default is 14 (days).