Xcode - Dark theme for the whole UI
Xcode's UI can be changed to completely dark with Xcode 10 running on macOS Mojave. Just go into System Preferences -> General -> Under Appearance -> Click the dark mode
You can reference to this article:
https://medium.com/@guilhermerambo/how-to-enable-real-dark-mode-on-os-x-macos-14966f9f7d24
Install a dark mode in system service,it make Xcode most control dark,it's not perfect, but works.
Hope Apple implement it someday.
In Preferences → Accessibility there is a checkbox Invert colors. Of course, it doesn't give a nice dark theme, but I found it quite useful when working in darkness. The whole system interface with this checkbox on looks funny.