Salesforce Colors extension
Simple extension that makes it easy to tell if you're currently on a Contact, Opportunity or Account record. All of these screens look very similar and it can be difficult to know which one you're currently viewing without scrolling all the way to the top to see the heading. This plugin changes the background color of each of the screens so you can recognize it no matter where you are in the record. It isn't customizable and it doesn't do any more than those three screens.