My plan for now is to detect when the current Gtk theme uses images and to fall back to some default colors in that case. Hopefully the telemetry being added in bug 1670145 indicates that a small enough proportion of the population does not use such themes. (After bug 1669368, we will unconditionally use the values of background-color on scrollbar parts from the Gtk theme CSS, but it's likely that those values are not appropriate when a background-image is also used.)