Themes for opn is created by CSS style sheets. (Several "themes" included)
It is very easy to figure out if you want to take a look and give it a shot.
Here's how.
1. FIRST COPY A THEME YOU LIKE
2. Rename the copied theme to name of your choice (whatever you want)
3. Open the newly named folder and look for the (style.css?) style sheet (it has a .css extension at the end of the name)
4. Read the coded comments the cool guys at OPN put in there for us newbies till you find the thing you want to change
5. Change till your heart is content. REMEMBER to make the theme you are working on the site theme so you can see the changes.
6. This gives the user (YOU) the best possible choice as opposed to (PostNuke, PHPNUKE) templates.
If you are unfamiliar with css (Cascading Style Sheets), then check out
www.sitepoint.com and search on css. It is pretty easy to understand once you get into it...
Hope this helps...
K