I am wondering if there's a way to change the colours used in the Matrix backend. We have a test environment and two seperate live ones, and often I'll be juggling all three at once. Because they all look identical when logged in as a backend admin, this opens up the scary possibility of doing something to the wrong site. If there was some obvious visual indication, e.g the test environment uses a two-tone blue bar instead of the purple colours used in the right-hand frame, this would reduce the risk!
I assume there's a system css file somewhere?
Feel free to direct me to some other thread if this has been discussed elsewhere, my numerous forum search attempts turned up nothing of relevance.
Thanks. 
editing css in /core/lib/web/css/edit.css should be all you need to do, I did not take too much care and I think I found the hex for the purle and find and replaced it…
Edit:
Something I just remembered (havn't tried it in anything above 3.16 yet) is to change the title for the dev install - change SQ_SYSTEM_LONG_NAME in /core/include/version.inc to something like 'Dev- Squiz MySource v'.SQ_SYSTEM_VERSION.' (Matrix)'. I Find it useful as you generally look at the title of your tab/window before clicking it.
I think the Squiz-purple is so sexy that no-one has ever wanted to change it before! :o
One thing we do is change the System Backend Suffix (_admin) for our different machines so we know what mode we are in.