I'm using PmWiki.
Here are my notes to myself on how I've customized it.
Relevant explanations at the PmWiki website:
The configuration file config.php is in the Local directory. I've added to that directory another file Books.php PmWiki looks for such a file when it loads pages from the Books group.
Books.php changes the skin to "Books" and changes the page logo.
The skin directory "Books" is located in the "skins" directory along with the default "ms" skin. The skin directory "Books" includes the skin template Books.tmpl which lays out the page (with left and right sidebars). The skin template calls the Books.css stylesheet which is in the same directory.