I use the default theme. I tried ZD-brown for a while but my eyes were beginning to feel a bit strained, so I went back. The default theme looks fine in my opinion anyway.
I use ZD Default because it was hard for me to read the text in the SB when people were still able to use colored text. So I got used to the theme. Maybe time to try out something else
I use ZD Brown. It's a lot easier on my eyes since I'm very sensitive to light. Plus I tend to spend a lot of time on ZD when it's dark out, so having something darker is easier on me.
My laptop's screen is very dim, even at full brightness, so I'm able to use the Default skin without my eyes melting. On any other device, though, I use ZD Brown.