Update 2
Okay, so I checked a little bit in the description of the "Dark Background
Switcher" add-on and they say:
"The plugin manipulates settings available in Preferences -> Content ->
Colors, if any problems, just uninstall it and edit the preferences therein
to your liking."
So I went in there and bam, now my backgrounds are black, my text is
"sandish" and my links are yellow. Nice!
But... When I do this I have to set the "Override the colors specified by the
page with your selections above" to "Always".
And it seems, when I do this, a fair share of the images disappear and there
are some texts I seemingly can't edit the style of (for example the texts
inside search bars). Also I am not able to change the style/color of for
example the background of this text box I am writing in right now in the
Trisquel forum.
Anyone got any hints or tips? Has anyone been messing around with this stuff
before?