Wanted: Theme with adaptable CSS

I’m looking for a nice, modern theme where the CSS styling does not use fixed sizes, that is, where the CSS is flexible enough to adust the width of the posts when the user makes the viewable area larger (like when you close your bookmark panel).

All the themes I’ve examined use CSS constructions like this:

width: 600 px;

rather than

width: 70%;

Any recommendations? Or am I being too ambitious thinking that someone would have built a theme using contemporary styling guidelines? I don’t mean to sound snarky. Many of themes have a great look — they just are not flexible.