Example: background-image: url(slide_bg.jpg)
Probably the most common change to a style sheet will be changing the background image. When you've created the new graphic add it to the folder containing the style sheets and other files specific to the presentation theme you're modifying. Open the appropriate style sheet (this will usually be title.css or slide.css) and look for the body selector near the top of the style sheet. Change the value inside the brackets to reference the new graphic.