You can set the CSS for individual wiki sites in a farm by adding a Fedwiki Theme Folder to the status folder of the site.
Wiki is sometimes too large. So I wandered what it would look like smaller. Here is some CSS that you can use to shrink your wiki.
Here, as part of a look at improving the use of CSS, we look at the style that is inlined by the client and plugins. Each plugin will get its own page.