FVWM-Crystal is a great looking mod and theme pack that is unintrusive and can co-exist with FVWM without altering its settings. There haven’t been any updates for a few years but it can still be compiled from source code and is available in the Debian repositories since Etch (4.0). Here’s the old home page. Now, it’s good to hear it’s actually not dead and you can get the latest release on box-look.org (direct link). It seems development has really picked up again and another one is in the pipeline. You can check the changelog on Sourceforge. Updates include new themes and bug fixes. FVWM is extremely light on resources.