/**
 * Custom CSS settings for GTK.
 * Check https://wiki.archlinux.org/title/GTK
 *       https://docs.gtk.org/gtk3/css-overview.html
 *       https://docs.gtk.org/gtk3/css-properties.html
 *       https://developer.gnome.org/documentation/tools/inspector.html
 *       https://blogs.gnome.org/mclasen/2014/05/06/tweaking-a-the-gtk-theme-using-css/
 */