That all depends on what you want to change exactly. If you post exact details then we can give you the css code changes you need to make. We will need a link to your site and full details of what you want changed.
Alternatively and the best way (as it will be quicker and you can experiment) is to use the excellent Firebug tool for Firefox. With this you can simple click on any element and you can see the exact css that controls it. You can even make live edits to see what it looks like before committing changes. In the case of JA template you will need to se css optimization to “No” in the template manager first or you would see the compressed css instead of the actual css files.