I like the look of this black on white theme that I made back when I was running Nodo. A couple of downsides though.
In messaging threads the colors of the messages are all the same, since the OS shows your message in a slightly darkened version of your theme color. I swear this wasn't the case back on the Mango beta, I remember it showing the messages as grey and black. I wish I had a screenshot of it, it looked really nice. At some point between beta and RTM, they changed it, so that it now darkens your theme color instead of lightening it.
I also was having problems replying to emails on Mango and eventually I figured out that it only happened when I was using my tweaked color themes. Since I was making the edits manually and not using a color tool, I must have screwed something up. Sadly, since I have an HTC phone I can't make registry edits on Mango so I never really use this theme anymore.