The reason I asked you was that I suppose you would know whether this problem
can be solved via the terminal or not.
You can certainly alter Abrowser's configuration files through command lines,
rather than through the Preferences or through about:config. But the result
would be the same.
I had never clicked on that "Colors..." button in the "Preferences". I see
the colors defined there are actually part of those I defined through the
preferences of the extension "Dark Background and Light Text". You may find
the use of that extension acceptable (you can know the exact color of any
pixel on your screen with tiny programs such as gcolor2, which is Trisquel's
repository).
But I am actually unsure whether you refer to the content of the Web page (as
I do in the paragraph above) or to the "chrome" (the rest of Abrowser's
window). In the latter case, you may find and install a Firefox theme of
your choice.
