They will probably never document it because they don't think anybody should change it. On the other hand, I can't stand invisible borders and I always need to

They will probably never document it because they don't think anybody should change it. On the other hand, I can't stand invisible borders and I always need to increase the border grab range. So just a good, highly-googleranked blogpost would be nice. I think the "correct" way is to copy the theme from /usr/share/themes to ~/.themes and make your changes there, then it won't break if the package manager updates them.