Over the years I found the perfect theme for working with Xcode.
It's based on the excellent solarized themeset: https://github.com/altercation/solarized
If you like dark coding themes, check it out!
Font is Meslo DZ, a customized version of Apple’s Menlo-Regular font. (Apache v2 licensed) https://github.com/andreberg/Meslo-Font
Copy the .dvtcolortheme files from this directory into ~/Library/Developer/Xcode/UserData/FontAndColorThemes - you may have to make this folder if you haven't changed your color themes. Restart Xcode, select new Theme in Color settings. That's it.
This is a nice complement to tweak the cursor in dark mode: https://github.com/egold/better-xcode-ibeam-cursor