The homepage in particular uses RGBA color values to add transparency to the text so it picks up the background tone, whether it’s a solid color or an image. Even the “black” text on the homepage is slightly transparent, which gives it just a hint of the background color. It softens the contrast a bit, and it reminds me of the effect when I used to overprint colors in print design.

CSS3 For Web Designers Paperback + eBook A Book Apart
Bulletproof Web Design Third Edition New Riders
Handcrafted CSS Book and DVD New Riders
Web Standards Solutions Special Edition Friends of Ed
Instapaper Icon design