It's no TouchFLO or SPB Mobile Shell, but it turns out you will be able theme Windows Phone 7 Series
... a little. Although we've only ever seen the classy "dark" theme with the black background, Microsoft just demoed a "light" theme with dark text on a white background. Earth-shattering, we know. Both themes also have selectable accent colors, but don't expect to go crazy here: your choices are red, orange, green and blue. We're hoping all of these options will be expanded over time, but for now it's not looking like a customizer's dream come true.
Windows Phone 7 Series will have themes... sort of