Fvwm is so nice, it's possible to customize everything from Color scheme to Mouse/Window behaviour. In Fvwm terms this is still mostly a theme, but given the flexibility themes can be very complex...
fvwm is like the pinnacle for obsessive configuration. I spent a couple hours over a weekend setting it up and thought it was really neat. The man pages and official website are very thorough, but I found this guide the most helpful getting started: