There are already several template sets available for pnForum:
Franky's phpBB Look
And the Cmods Devs already build some Themes with templates for pnForum
For example: AphroditeXTE
Other Themes with pnForum templates:
Download them: http://www.cmods-dev.de/module-Downloads.htm
See them: http://demo.cmods-dev.de/ (Use the ThemeSwitcher)
The themes contains a folder /themes/ThatTheme/templates/modules/pnForum copy that into your own theme at /themes/ThatTheme/templates/modules/
This opens a popup displaying all available variables. If you don't know how to use them you can always look into the backed up version of the template.
BTW: Frank Schummertz, maintainer of pnForum, spent alot of time creating seminatic HTML for the forum. But he's looking for someone who's good in creating a nice CSS for pnForum.