CSS and Background

Today's quest is to figure out how to make a new and unique blog background with animations, bells and whistles and so forth and break away from the prestructured blogger templates that have made me feel locked in and confined for years. So far I'm drowning in computer lingo but I did just find out that CSS stands for Cascading Style Sheets. What I haven't figured out is how to download a template to my hard drive and then add my own background photo. That is quest #1.