prefs: Add pref to use dark theme variant

Now that there is gtk+ API to reset the setting, let's bring back
the pref.

Note that the pref defaults to following the global pref. If your
distribution has set a different default (e.g. by using a gsettings
override), DO NOT complain to upstream about this!

This reverts commit 5f6c514a.

https://bugzilla.gnome.org/show_bug.cgi?id=649358
https://bugzilla.gnome.org/show_bug.cgi?id=707206
https://bugzilla.gnome.org/show_bug.cgi?id=742040
https://bugzilla.gnome.org/show_bug.cgi?id=742333
https://bugzilla.gnome.org/show_bug.cgi?id=755008