Delete background-color of th

If you look closer, you would find that the title th element covers the separator line, the border of next code th element. I found it's caused by a useful-less background-color, so I delete it.
Merge pull request #517 from RixTox/patch-1

CSS clean-up to remove seam between code and text.
Merge remote-tracking branch 'remotes/pivotal-original/gh-pages' into gh-pages