How to force Xcode(or any other app) to be on dark theme while macOS Mojave's setting is on light mode
Open Terminal and run the following: 

defaults write com.apple.dt.Xcode NSRequiresAquaSystemAppearance -bool FALSE


source: https://medium.com/@pawelurbanowicz/boost-your-productivity-with-macos-mojave-enable-dark-mode-for-xcode-only-33d86e452ea9