I downloaded xcode 10 beta and installed it on my macbook which is running on macOs 10.13.4 High Sierra. I tried to enable dark mode on xcode 10, but i could find how to do
Only for macOS Mojave 10.14 and onwards..
If you want to use Xcode in dark mode while maintaining a different system wide mode or generally modify the mode per app I can recommend Gray (https://github.com/zenangst/Gray). Gray is a simple tool providing an interface for application-specific user configurations.