Someone should build a standard vocabulary of CSS elements for site customization. That has probably been said before, but for web apps to become even more integrated with user desktops, they need to be as easily themable as their desktop environment. This is not just a question of making everything pretty and slick, but a real usability issue. Needless change is distracting to users and makes new tools hard to learn.

I think this can be accomplished using the right blend of technology and advocacy. From the technology end, participating websites can include a stylesheet reference to http://usercss.org/my.css which would send the user's chosen theme (stored in a browser cookie). If the user hasn't yet registered with usercss.org, it should send the default theme for the referring site. Each site can provide a "choose theme" link that would display a co-branded, but consistently-designed configuration tool.

On the other hand, this usercss.org would also be in a great position to collect site viewing statistics and provide single sign-on, and thus subject to much of the same skepticism. Maybe it really isn't feasible.