I wrote a really simple theme for WordPress called, in a flash of brilliant creativity, Ultrasimple.
After reading dozens of posts by people wanting to include a blog in an IFRAME, something I wanted to do myself sometime back but never found a good solution for before deciding that there were better options (RSS readers specifically), I cobbled together a WordPress theme that fits everything in a single 350 pixel column with a white background. Changing the color is a matter of changing the colors in the style sheet (style.css) for whichever elements you want to change.
I included the bare bones “meta” information to login to site administation and RSS feeds in the footer instead of the sidebar. The header is just a black title linked to the blog URL – simple so it’s simple to modify.
It’s nothing special – it’s not supposed to be – but it is valid XHTML and valid CSS.
Tested at My Library but not currently in use there, sorry for the confusion.