punbb>styles>oxygen>oxygen.css and oxygen_cs.css
and you open it in (for example) PSPad. it's simple css file.
if you don't know css-language, it will be hard to change style 
1 2009-10-04 07:32
Re: few questions about editing punbb's style (5 replies, posted in PunBB 1.3 additions)
2 2009-10-02 20:30
Topic: few questions about editing punbb's style (5 replies, posted in PunBB 1.3 additions)
I have some problems with PunBB on my site.
i use Oxygen style (standard), which i have modificated (i've added logo, background and changed colours)
1. How i can delete whitespaces between footer, main part of forum (with topics etc.) and menu?
2. How I can add my page's menu on the top of the board? (<a href= "http:// ....">page 2</a>)
3. How i can add the footer under the forum? footer from page, with one link
thanks