Not usually. The reason is that most fixed width themes are fixed width because there is a header graphic in the design. The way around this I guess is to take that header graphic, or any other, and create an x-tiled band of color behind it. Then you can make that theme more fluid. Use percentage widths for the layout divs. And set a base font in body and use percentages or ems elsewhere.
Here’s a discussion I ran across just the other day about fixed width versus fluid width layouts. It’s a little over a year old, but still relevant.
http://www.pat-burt.com/web-usability/fixed-vs-fluid-website-layouts
if you aren’t using a bunch of background graphics you can change width pixel count to percentages.
