Use system colors for preview window
Ubuntu 18.04.1, GTK 3.22.20, Gnome 3.28.2.
On my system I use Adwaita theme, i.e. a bright theme, not the dark one. Nevertheless the preview window is dark theme. I know that there is a tendency to make 'media' apps (like a video player) use dark themes. I don't like it as for me it overrides my preferences.
Edited by Ghost User