It appears something about my existing profile was incompatible with the new version, causing a number of these strange issues. I didn't have a UserChrome.css,

It appears something about my existing profile was incompatible with the new version, causing a number of these strange issues. I didn't have a UserChrome.css, nor was I using a custom theme, so I don't know what it could have been, but creating a new profile at least dealt with the strange colour issues. And thank you for that drag space tip. Definitely part of what I was looking for. I just assumed it toggled the ability to drag whitespace around while customizing!