The theme plugin allows easily applying a theme to your wiki, by
theme setting in the setup file with the name of a theme
to use. The themes you can choose from are all subdirectories, typically
/usr/share/ikiwiki/themes/. See themes for an overview
of the themes included in ikiwiki and the theme market for third party themes.
You can set the theme via the theme option in your config file (after
enabling the plugin). Refresh the wiki (with
ikiwiki --setup <file>)
after changing it to see the changes.