I just read the discussion that went on about the Themes for Perlmonks Quest and I think that there might be a way to make personalized theming easy enough for users without it becoming a nightmare for admins.
This site's pages are extremely well structured, which makes them excellent candidates for styling using CSS. I believe most graphical web browsers implement a sufficient subset of CSS for it to be a workable option. Plus, it would make the pages lighter (by eliminating at least all the font tags and the bgcolor attributes). CSS (Cascading Style Sheets) are simple enough and I think almost anyone can easily learn how to use them.
On the preferences node, the user could be presented with a simple textarea containing some basic CSS code to edit (several templates could be available). All widgets types on Perlmonks would get a class attribute (eg class='rhsBox') and perhaps in some cases an id attribute too (eg id='rhsBoxInfo'), the latter allowing one to style the information box differently from the other boxes.
Once a user would have saved a personalized style sheet, that style sheet would be the one served to him through a simple link tag in the head section of each page. As an added advantage, this would make theme contribution much easier.
The main reason I'm thinking about this is less for fancy additions than for readability. I'd like to be able to truly fine tune fonts in various places, as well as some color contrasts. Making all those options available in any other way would be painful. Using simple user editable style sheets I think (hope) everyone can be happy.