:) Yeah, we will be providing some sort of theme options eventually, but honestly I think having a dark background will stick. You're sort of crazy if you want an in-your-face white and bright experience. I don't see the concept of light text on a dark background changing much.
As for dimensions, I have been making them as big as I can, basically as big as the original image source allows. This way they can be used as backgrounds too, if desired, eventually. The aspect ratio works better though, of course, when they are wider than they are taller. I should also note that they should all be 24-bit transparent PNG images.
Yes, the stroke is because, for example, the red of the SNES logo is too hard to read against the background. This is unfortunately common.
Awesome to hear you're hopping on board, @bd00. Thanks a million.