Rcss is an implementation of CSS SSC for Rails framework.
(It’s interesting to see that the description says “implements CSS-SSC in Ruby”, while the instructions specifically apply to Ruby on Rails. If confusion is growing between Ruby and Rails, does that make Rails the de facto framework for Ruby on the web?)
I’m not a big fan of server-side CSS as a tool for webdesigner, the point to them seems to lie mostly in customisation for end users. The reason is that it’s a tool to help with mass stylesheet modifications, a use case that is rather unusual in the real world for a single designer or team, and for which server-side CSS are a more like a punctual burden than a long term solution.
On the other hand, for people whose only will or technical ability is to change the fonts and colors used on their site, having a tool like Rcss modify some strings helps with customisability.
For example, a weblog platform could, in addition to a choice of templates, provide a way for end users to customise said their chosen template’s stylesheet; it would be cached as a static file. So aunt Maggie could use dear old Comic Sans for her headings and the host wouldn’t need to rebuild all her template files (like 20six currenly does, for instance).
Heck, themes for standalone apps could be distributed with a server-side CSS version, with defaults that could be overridden by the app (hello WP/MT/whatever themers). That would be the end of support threads like “I like [insert popular theme] but I want it to use blue Helvetica instead of that ugly font”. Less noise, more happiness : what are we waiting for?