I use the Space Engine's theme (that can be considered "Retro", if I'm not wrong), but I've personally edited it
(for example I've changed the main menu buttons color).
Eventually I can publish it if you don't mind.EDIT
: Well, I already know I'm going to forgot so I'm gonna do that in advance. I remember I've changed the appearance of the buttons, but now I realized I've edited the .png file for doing that, and changing the .png file will reflect the edits on all the other "retro" skins of the game, and... well, they're violet/blue so they doesn't really fit to other colors...
So, yeah, it's not going to help a lot, unless if I paint them gray/black again. I also changed other features too, windows are slightly more transparent (knowing that the background will be blurred) and "close" button is now reddish, but basically it's the retro skin with rounded and violet buttons and some minor changes. It would look a lot better if I had the time to put a proper background (with right colors) and a proper logo.