Oh, you know, I had another thought: If you do choose to have a style directory structure, it might be a nice touch to have the forum parse a very simple text file (included in each theme, and given a certain name and location in the style directory) to give the designer a little credit in the footer somewhere. The text file might even be just one line, or parsed down to the first 256 characters to prevent it from being abused. It would be a nice little nod to theme designers to have a means for them to place a small link on the bottom of the page like they can with that other forum.
Re: markup
I don't mean quirks as a bad thing, the mark-up is actually quite nice. But there are a few areas that I find peculiar, and that's why I say "quirky."
For example, the use of non-breaking spaces. In the <dd class="usercontacts"> there are nbsps insterted before the web address, and I'd think that semantically they might be separate definitions, but at the very least, the nbsp needn't be there, some margin or padding would do the trick just as well.
Along the same lines, it's remarkably helpful to stylize links when either the enclosed text is wrapped in its own span or the link is wrapped in its own container. The end result allows a designer to use the technique popularized by alistapart's sliding doors article. I've only created one skin (and it's in progress, I'll let you know when it goes live) and I've found that most links are able to be styled with the sliding doors approach since most are in lists or other wrappers, but user contacts is a notable exception.
here's another that I find quirky:
<div id="brdwelcome" class="inbox">
<p class="conl">Logged in as <strong>roo</strong><br />Last visit: Today 12:50:10</p>
...
<div class="clearer"></div>
</div>
I think the break tag in that passage should be removed and instead have the Last visit part be wrapped in its own container. A span or even its own paragraph. It's workable, however. I was able to single out that break and apply display: none; to it, but it is quirky to have a break tag sitting there for reasons that appear mostly presentational.
I find it a little odd that there is a bar | between Report, Quote, etc. I don't feel that it should be a part of the markup as it requires a designer to remove it if they choose an alternate means of styling that link. The same effect can be achieved by adding border to one side of each list item, and by adding a class to either the first or last item in that list to which there is no border applied.
Also, it might be a nice feature to identify individual links. The main nav might get an id on each <li> like <li id="index">, <li id="userlist">, etc. The list at the bottom of each post might also be given classes <li class="report">, <li class="quote">, etc. As it is now, I've applied different styles to each link by using css selectors that aren't supported in WinIE. Not that I particularly care about WinIE, but having identifying attributes on those list items would make styling easier and more flexible.
Again, as it is, the markup is very workable, and the tags and classes are very well thought out and applied, but these are a few things I encountered. Overall, it does a bang-up job of giving a designer everything he needs to style the forum, and again, I was able to work past nearly every one of these issues. I didn't want to get into it earlier because I feel that I'm largely nit-picking, but you did ask. 
)
