css styling and theme styling

Could someone help me find or tell me where to find examples of how to change the css styling for buddypress. I am using twentyeleven theme on wordpress with buddypress widget theme. I have firebug installed and I am trying change things like background color of the content part of the page as well as the sidebar, change header size, and how to add tables so they become part of the content part of the page and create a scroll so the page reacts right. I dont want to use table plugins, I want to add them where exactly I want them. When I add them and position them through css they dont work right in the content area. I need to know how things work so I can learn to make nice styles.