Well, first theme for me that wasn’t just changing stuff in someone else’s.
Spiffty is best described, I think, as a two-column light-blue and green theme. As far as I’m concerned, it has a somewhat modular feel to its appearance. The sidebar is easily customized, as well as the header/menu tabs.
There’s built-in support for tags through UltimateTagWarrior, but it’s not required that you install it. If you decide not to use UTW, they’ll just not be rendered.
It’s written using XHTML 1.0 Strict and uses about 18kb worth of images.