One way to inch into this safely would be to start embedding <link type="text/css" rel="stylesheet" ref="/css.pl?node_id=99999"> into pages. In the default case (the user with that node_id has no css-sheet specified), css.pl could emit a stylesheet that's appropriate for the target browser.

A simple next step would be to offer a palette of stylesheet options, and let users choose one. The more complicated (but still relatively easy in the grand scheme of things) step would be to let monks edit theirs via user settings.

The space requirement for user-editable stylesheets probably pales in comparison to the space eaten up by monk pictures.