The Spiffy Theme Guide
From Spiffy Stores Knowledge Base
Spiffy Stores use a templating system that incorporates special "liquid" tags that make the dynamic content appear in the templates. All templates use standard HTML & CSS for the layout, and can be opened in any program designed for editing or creating web pages.
A "Theme" is simply all of these HTML & CSS templates along with their images all zipped up in a zip file.
To create your own theme, it's a LOT easier to choose a theme from the theme gallery that's closest to the design you want to achieve, apply it to your site, then download it.