Whenever you want to know how anything is styled, you need to view the source of the page, find the markup you are interested in styling and see what css classes are assigned to it or around it. Once you know that you can find or create the right css to achieve your desired result.
Typically in mojoportal skins anything to do with layout is in style-layout.css of your skin and anything to do with images is often in style-images.css.
Please see the notes about skinning. Its not comprehensive documentation but it has a lot of important information. The main thing is learning css and html.
Hope it helps,