Using CSS with ikiwiki
Ikiwiki comes with two CSS stylesheets: style.css and local.css. The idea is to customize the second one, overriding the first one and defining brand new rendering rules.
While ikiwiki's default use of stylesheets is intentionally quite plain and minimalistic, CSS allows creating any kind of look you can dream up.
The theme plugin provides some prepackaged themes in an easy to use way.
The css market page is an attempt to collect user contributed local.css files.
Per-page CSS
The meta plugin can be used to add additional style sheets to a page.
The localstyle plugin can be used to override the toplevel local.css for a whole section of the wiki.