Xcode environment not dark in Mojave beta's dark mode I feel really stupid for asking this, but here goes:

Is there a button or setting somewhere to make Xcode use dark mode for its environment in the Mojave Beta?

Searching for this renders a lot of questions about dark mode in High Sierra, or how to make your programs adjust to dark mode, but what I want is for Xcode itself to use dark mode, which just doesn't happen on my 2015 MBP with Mojave Beta. I guess Xcode, being an Apple product and all, should detect that OS X is in Dark Mode and adjust accordingly?
For Dark Mode to work, I needed Xcode 10 beta. Thanks to user128998 for helping me out!