CSS and Overlays

This might seem obvious but if you need to modify the CSS for your admin pages which appear in overlays, you will be needing to create a subtheme.

Assuming you use Seven for your admin theme, create a subtheme based on Seven, let's say you call it SevenUp, and add your own CSS overrides, then change the default admin theme to SevenUp.

You'll get all the Seven settings and your overrides.

However when you enable your theme you'll have to go to the Block admin page and remove all the blocks which have now added themselves.