Apparently, if you change the forum skin to mobile, and then while in the mobile skin try to enter the UserCP and change it back to default (or grey, my choice), the forum skin option is not available!
I used the back button on my browser and fixed it, but I’d assume (unless I’m missing something), that if someone did this, closed their browser, and then wanted to fix it, they couldn’t, and it would be incredibly hilarious.