Some more fixes in trackhacks.css

[bmispelon]

I've split the PR over several independent commits, addressing the 4 following issues:

• Active comment has yellow background #164 and Shadow-looking background on comments #163 in 0b7d045
• [styling] Buttons have text-shadow.  #153 in f63e17b
• [styling] Too much whitespace between mainnav and content #152 in b80dec5
• Api section table UI overflowing outside. #144 in 7178278

Here's the before/after for the changes to the comment header:

[felixxm]

@bmispelon Thanks 👍