Yours is not a simple question. You need to describe in some considerable detail what you want changed in your forum.
Some things are simply not possible without rewriting the NAML code that generates the HTML that underlies your forum, so it's not just a matter of understanding CSS but also the HTML on which it hangs.
The easiest approach might be to find a forum with the design you like. Look in the source code for that site and find the contents the file:
Pasting that code into the CSS box on your forum will probably reproduce the design of source forum - but it does depend on other customisations that may have been made.
As I say, yours is not a simple request.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me!