Are you feeding any protein or calcium beyond fruits and vegs? Diet is so important for maintaining proper health and your girls need protein nightly. Google BML, OHPW, TPG -- those are staple diets that seem to be the most popular. You can also look along the top yellow bar on this website for more diet choices. You have many to choose from, but it is integral they have one.
Beyond diet, lack of cleaning cages/pouches/wheels is another common cause for staining. The wheel in particular, as they eliminate while they run, then continue to run and that waste can rub off onto them. On the flip side, cleaning TOO often can have the opposite effect, and cause them to go crazy over marking to make their fresh clean cage smell like them. Females don't mark as heavily as males, but they still do and will.
You'll have to find a schedule for cleaning that works well for you, based on your gliders, and your cage size. I'll tell you mine, to give you an idea.
Every day: wipe drop tray, wipe down floor grate with vinegar/water mix. Wash glider kitchen and food bowls, refill water bottle.
Every 4 days: quick wipe of wheel, cage bars. Swap pouch for new one.
Every week: wash either all bedding, or all toys.
Every month: steam clean cage itself
When I first got gliders, I cleaned everything all at once, every 4 days. Needless to say, my place was stink city because I was over cleaning!