This is a great idea and it's working as expected so far.

Lightweight and simple. Visually identifies environments for worry free testing/development.

I didn't know I was looking for it until I saw it!

This is simply awesome!

This extension can be used in another way: set your environment as keywords surrounded with asterisks such as *mozilla* or *addons* and then you can quickly guess the topic of an active tab by looking at the theme color :-)