Details
Would be great to give a instance different background color and/or top left logo?
Event Timeline
You can choose the header background color via the "ui.header-color" config value. Not exactly what you are looking for but it can help.
Answers
Event Timeline
It would be good, as we have already three Fabricator instances running plus monitoring the upstream one. Some background color indication would be great to know where you are actually... ;-) a setting for some sensible CSS values would be good.
There is currently an option to set the color of the header bar. I use this to help differentiate between our production install and the test/dev install (and also to help me differentiate between the install here at secure.phabricator.com ;p).
See ui.header-color under /config/group/ui/.