If you want more control over the look of Workbench you should install VisualPrefs and FullPalette, and hopefully someone can help you out with a MUI keyfile because it's really nice being able to configure MUI too.
A friend of mine is working on a theme manager program for Workbench which lets you make new themes based on your current presets and save them, and you'll also be able to download and install new themes to choose from (which I'll make a few of to try at first). Hopefully she finishes it soon, but she's still teaching herself to program and is to afraid to post on forums anymore. I've seen it running and it works, it just needs some bugs ironed out.
Until then you should still install these programs:
http://aminet.net/package/util/wb/VisualPrefs
http://aminet.net/package/util/wb/FullPalette22