stylesheet.css: improve dark mode firefox display
stylesheet.css: improve dark mode firefox display

- Make the search input background white even in dark mode.

- Remove the horizontal scrollbar on the nav unless the screen is small.

Fixes b/143611569.

Fixes b/143613221.

Change-Id: I770b2a7fd9594d3d1152e9d8bcc8558d36affdc4
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/592089
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Alexander Rakoczy <cc99db1321cf6c6b239cafe26d8f2eb6f93aaa8d@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite