I am finding it to be a bug when using transparent logoff dialogue backgrounds. Dunno if it is new or not. With most skins you don't notice the dark outline around the button but it is there. In your case the skin really exaggerates the issue. Every skin I tested with a transparent logoff dialogue has the same problem. I tested alot of skins and had the same result.
I am hoping Neil or Adam gets a peek at this and can confirm a bug.
This is the first time this has happened. I have been using transparent logoffs for awhile now, along with transparent buttons, and haven't had that issue. In this case, the button actually changed color! The only thing I could think of was that the logoff doesn't have a bottom edge on it. I even went and changed the logoff to a "regular" bottom, and it didn't help.
I am still using Windowblinds 5.1. I haven't updraded it yet.