I think the themes are weird at it makes me feel like part of my wallpaper is sticking out, not to mention the buttons become not as seeable. At least that is what I saw on my sisters computer, not really my style :P
I -used- to use the Acid Burn skin for firefox, but it doesn't work in 3.x. So I'm currently using NASA Night Launch, though I may switch back to Pitch Black. I will definitely try Personas though.