With my color profile set to a black background, RoxTerm shows a brief flash of white when the terminal window opens, and there are white edges visible briefly while resizing the window larger. This is due to the white background color of the GtkWindow widget (at least with my GTK theme), because it resizes and draws before the VTE widget gets a chance to.

I was able to override the background color to avoid this by creating ~/.config/gtk-3.0/gtk.css containing:

GtkWindow {
    background-color: black;
}

It would be nice if RoxTerm could just do a gtk_widget_modify_bg or gtk_widget_override_background_color to match the terminal's background color, instead.

Overriding like that in gtk.css is a bad idea, since it changes all widget backgrounds. Instead, I made a patch for roxterm to set the color, and will create a feature request ticket with that patch.