Wiki customization: change CSS won't save

Wiki customization: change CSS won't save

I've tried changing the CSS url at the bottom of Settings->Customizations, changing it to both an uploaded file and a web URL.  In either case, the Save button does nothing and it retains the css url that was there before.

How do I change the CSS to my own file, retaining the ability to change it back to the existing one if something goes wrong?  I also have downloaded the existing css, as a backup.