If the gauges are backlit when designed, then they should backlight when the cockpit is dark. With .gau and XML gauges, there is a 'luminous' switch that is normally incorporated, that syncs with the 'luminous' command in the [color] section of the panel.cfg file, to provide the backlighting colour shade (the figures are the RGB values).

If the gauges aren't illuminating, then maybe the original gauge designer didn't make them luminous. You can check this out by temporarily replacing one of the gauges with a stock M$ gauge file which will illuminate. XML files can be amended to incorporate this switch fairly easily, but decompiling .gau files is a different story, I think.

Hope this helps.