It's Shift+F1 darker, and Shift+F2 lighter.
While I'm on the subject, I get the impression that the importance of interface color is underestimated. It actually has a huge impact on the perceived brightness of the image on-screen. The new(ish) dark interface is very easy on the eyes, and great for web and video work - IOW images destined for screen. But for print you can easily be tricked by the dark environment and end up with the classic "my prints are too dark". I would say that the old and boring light interface gives you a much more reliable preview of what the finished print will look like.
The same consideration applies to Lightroom BTW. Here you can't even choose, except you do have the option for a light main window backdrop, which helps (although the whole UI gets messy and inconsistent).