Xcode, change background color of selection
Have you checked the Font and Color Preferences?

You can customize the appearance of source code and console text by changing their colors and fonts in Fonts &amp; Colors preferences. 


Choose Xcode > Preferences and click Fonts &amp; Colors.
Click either Source Editor or Console near the top of the preferences window.
Select a theme in the left column.
Select a syntax category in the detail area.




Then if you want, you can also customize colors for the: background, selection, cursor of the source editor and the console.