Actually that wasn't the expected outcome, which was: restore dark
or system-wide (also dark) theme for the toolbar of DevTools (incl.
error console). Originally, this was the case but then, there has
been some change ruining the light theme, most likely this:
https://bug957117.bugzilla.mozilla.org/attachment.cgi?id=8368131
Proper solution will likely be to edit user's userChrome.css
overriding some of the values (especially search for #ebeced).
This reverts commit 85245d779883cdbc96557b2a9656b57d932e6737.