header: add padding to underline of selected topics

[thomesh]

additionally, turn active section black

[sesneaky]

Conform to page 5
https://trello.com/c/iFGxnc9M/16-style-guide-v1

[hamoid]

I tried to fix this issue with 64c8070

But the style guide does not include design examples for the mobile version of the web, in which the menu is vertical.

Also, the highlighting of the current section does not work when clicking on menu items or when scrolling down the page.

If this change is considered an improvement it can be merged into master.