Although, this represents what is probably the majority of browsers in common use, where does this leave users of older browsers.
If you look at a large number of PHP-Nuke themes, they look absolutely horrible in Netscape Versions 4/4.7. While I applaud the implementation of CSS in Post-Nuke, this does not represent the best option for all end users. As a web designer, I am often tasked with dumbing down a real wizbang site so that it look similar in different browsers (or at least degrades gracefully for older browsers).
For this reason I propose an alternate implementation for themes. What if developers had more that one way in which they could utilize themes on their site. What if there were a configurable option to allow the site to select a theme dynamically based on the end user's browser. This way you could develop a theme in the best and most efficient way possible for a particular browser.
I believe that this idea opens a wide gamut of possibilities. You could develop an IE theme that takes advantage of all that browser has to offer without worrying how bad it may look in Netscape. You could develop a Netscape 4/4.7 theme to compensate for those CSS attributes that these versions did not incorporate. You could even develop a text only theme that would be very useful for people that need the sites they visit to be handicap accessible. While I know that this last scenario could be factored into nearly any design, nothing comes close, usability wise, to a solution completely aimed at that demographic.