I hate to say this about the framework I love, but it's true. Anyone who's ever spent more than a few minutes developing skins or modules knows it, but so far it hasn't inspired anyone to step up to the plate and take on refactoring/refining the CSS into a more workable state. Maybe it's not as big a deal as I think, but I'm tired of looking through the CSS that gets sent downstream and comparing it to other frameworks that do a tremendously better job of producing clean code. I'm not a huge fan of reading other people's rants if they're not willing to offer suggestions, so here goes. Please feel free to agree or disagree with any of these points- that's what comments are for.
Create a consistent CSS naming scheme
Enforce (or at least strongly recommend) that developers use a distinct namespace when creating the CSS for their modules. Take, for instance, the DotNetNuke Blog module. IMO, all of the CSS entries should be proceeded by a distinct selector (i.e. .dnnblog) that all of the blog sub-modules are wrapped in. If I want every blog module on my site to have a certain background image, or use a certain typeface, I should be able to do it in a straight-forward manner.
Zero out default.css
And then add back in any of the available CSS reset files out there. Yahoo's, Eric Meyers's, any of them will do. After that, define basic styles: headers, paragraphs, etc., using the minimum amount of CSS. Don't define font-family anywhere, especially not for every textual element possible.
If DNN wants the default skin to look polished and awesome, great! Keep it all in the skin.css-that's what it's there for. But don't make me hunt down and override items, especially when I'm banging my head against the wall because a div > div > ul > li is displaying in a different font than div > ul > li (remove all font-family definitions from default.css and the problem disappears).
Create an admin.css
All of the administration classes, both in the control panel up top and in the admin pages, should be rolled into an admin.css file that is only included if you're logged in as an admin. There is no reason that CSS for the control panel is sent down with every page view. Also, create a dnnadmin class that it's all wrapped in.
Get it done for DNN v6
The DotNetNuke skinning team lead, Timo Breumelhof, mentioned that this work was deliberately avoided so as not to introduce any breaking changes for existing installations. Considering DNN5 broke existing skins and modules, wouldn't that have been the perfect time to also break the CSS? I mean, if a developer already has to go back in and update class names, replace depreciated functions, etc., wouldn't that be the perfect time to do this? No one ever said a cross-version upgrade would go smoothly (heck, even a lot of minor upgrades don't go smoothly…), but at some point, we've got to bite the bullet and move on.
Allow custom classes to be defined in the module settings
For instance, in a certain text module, let me go into the settings and assign a class to that module that would be injected into the container wrapper. This is less important with the text module since I could just go in and wrap my text in a classed div, but say I want two surveys on the same page that look entirely different. True, that can be done now using separate containers, but it seems like it would be a nice advanced option to have.
Update (3/14/2011)
While I haven't given up on DNN, the lack of designer friendliness has led me into the arms of Umbraco. I'm writing this update as I'm migrating my old blog entries over, and I must say-I'm impressed with the Umbraco I see now versus what I tried out two years ago.