So I changed the global theme of my YAF site to one of the default supplied themes ... can't remember it's name, was it "Fancy" or something like that. So now in my admin panels all I see is the YAF logo and a blue background ... that's it, no text, no controls, so no way to change the theme back to something that doesn't render my site totally useless! Any suggestions?

Which version of yaf are you using?

2.1.2, latest available from my ISP's automated installer.

Yes sorry that's the latest release available via the web platform installer. It takes a couple of weeks and until the latest release 2.2.1 is approved by Microsoft.

Until the. You could upgrade manually by download of the latest version directly from github

https://github.com/YAFNET/YAFNET/releases/download/v2.2.1/YAF-v2.2.1.0-Upgrade.zip 

The upgrade process is straight forward

https://github.com/YAFNET/YAFNET/wiki/Upgrade-(from-Version-1.9.5.5-or-above) 

That's OK, I figured out how to grab the updated .js file from GitHub, put it in place, and that allowed me to get in to disable the theme.