I'm done with this. If someone wants to maintain it send me a note and I'll transfer the repo. Or just fork it, or whatever.
It isn't finished yet, but you can see it in action here.
Make sure the files are in a folder if they're not already (ex: a folder called "pretty-simple")
Put the folder in your GNU social's '/local/theme' directory (create it if it doesn't exist)
Tell your GNU social instance to use the theme by adding the following to "/config.php":
$config['site']['theme'] = 'pretty-simple';