If I apply the patch below to gnome-control-center to switch it to dark theme and start it, the separators in the main view (icon list of panels) are not shown. If you unfocus the control center window, they appear, and disappear again as soon as the window is focused. Things are working as expected (separator always appearing) in the non-dark theme.

    diff --git a/shell/control-center.c b/shell/control-center.c
    index a3f020b..3595eb6 100644
    --- a/shell/control-center.c
    +++ b/shell/control-center.c
    @@ -253,6 +253,9 @@ main (int argc, char **argv)
    g_signal_connect (application, "command-line",
    G_CALLBACK (application_command_line_cb), shell);
    + GtkSettings *settings;
    + settings = gtk_settings_get_default ();
    + g_object_set (G_OBJECT (settings), "gtk-application-prefer-dark-theme", TRUE, NULL);
    status = g_application_run (G_APPLICATION (application), argc, argv);
    g_object_unref (application);