There are a lot of themes here that allow you to add custom header images, and also allow you to change the background color, or even put in a background image without needing the CSS upgrade.
If you go to appearance > themes in your dashboard and then click on the "feature filter" link that is to the right of the search box, you can select "custom header" and/or "custom background" to find which themes have one or the other or both.
At the top of the CSS forum, there are a couple stickies (in the yellow section) that have tutorials and resources you can look over. One thing to keep in mind is that CSS is always theme specific. Each author decides how they are going to arrange things and what they are going to call them, so the CSS customizations from one theme will not work with another theme.
The base font size and color are typically always defined in "body." Header or title elements are typically defined as h1, h2, h3, h4, etc. and defined separately. For other page elements that need different fonts/sizes/colors, those are typically defined in the CSS for the element itself (if needed).
The main thing to get to know and understand is how the markup (XHTML) and the CSS relate to one another and that is done by viewing the page source in your browser and looking at the HTML in the "body" section and then looking at the IDs and Classes for those elements in the CSS.