Just a little thing, but it would be nice to be able to change the colour of the header bar (either the black or the blue one, or both). I'm really looking for a way to distinguish visually between the UAT and production instances, but a bit of branding probably would be popular.

Advanced option - a CSS customisation page could give the same result and add more possibilities at the same time (but from bitter experience, disable the customisations on the page to control it. At least that way you can unbreak a mess!).