Description of problem:
wrong style. partially using gtk style partially using css style.

Version-Release number of selected component (if applicable):
firefox --version
Mozilla Firefox 44.0

How reproducible:
I use dark gtk theme.
Go to one site from list below
youtube.com (the background of the search field is white(css) color font is white(gtk).
bugzilla.redhat.com (the background of the search field is gray(gtk) color font is black(css).)
valadoc.org (there is button in search field. it don't show but it work)

Steps to Reproduce:

Actual results:


Expected results:


Additional info: