Greetings!
Recently I went back to other 3D applications, namely Maya, and realized how easy on the eyes the UI colors were. So I went ahead and did my best to modify the ZBrush UI Colors. I did 5 variations (2 are just a slightly darker contrast). Personally I like #5 the best and use it as my default but I thought I would share the other ones.
The biggest change in these themes is that I removed the gradients and shadows as much as possible. Those might have looked cool at some time but removing them made it more easy on my aging eyes.
In order to apply the themes go to Preferences menu then select iColors and then Load Ui Colors.
Unfortunately, for the viewport background colors you need to go to Document menu and change the Back, Border, and Border 2 colors there. I used the same color for Border and Border 2. Also set the Range to 0. The RGB numbers are as follows:
Back = R48 G48 B48
Border = R61 G62 B70
Lastly, remember to go to Preferences, Config and hit the Store Config button so that the colors remain after you restart ZBrush.