This is a slightly modified version of gnumdk's kdm version of Tux-Mania. Original link can be found here:
There was a box with a plain black background around the login fields, inside the nice painted box you see on the thumbnails. I made it semi-transparent (67%), just like all the other black fields around are. Have fun :).
PS: I didn't find gnumdk's e-mail addy, therefore I had to create a new post. If anyone can give it to me, I'll mail him my little update so that there's only one post with this theme in it ;).