It's extremely common on websites to see solid colors spanning some portion of the top and bottom of the website, much like can be seen on some of the stack sites. Stackoverflow is a good example, it even has the significantly expanded bottom that also seems common.
It's a simple feature and surely doesn't have a greatly complicated origin story, but still, at one point I didn't see see this motif and now I see it on about half of all websites.
What changed? Was there a particularly influential website that did this or a particularly influential document that suggested it? Or is it just a coincidence since it's simple and looks nice?
EDIT To clarify what I mean, here are some examples. They tend to have solid colors spanning the entire page, a larger different color block at the bottom, and navigation elements arranged horizontally. How I "feel things used to be" is that there was typically a menu to the left and a single solid color background.
(I couldn't think of anything else so I just googled web design for the last few and picked ones that show the motif.)