The information given to you by store is not correct; these frogs require daytime temps of 85F and can drop to around 75F at night. Here is a well written Care Article with lot's of good information:
Frog Forum - White's Tree Frog Care - Litoria caerulea. Humidity is not as important as ventilation with these guys and 50-60% is OK and would avoid anything above 75%. There are different set-ups you can build for a White and depending which way you go might need to spray enclosure maybe once a day. Can spray enclosure with distilled water to avoid water marks but frogs water dish should be dechlorinated tap or bottled spring.
A 12X12X18 in. would do for a baby; but adults are large frogs and require something like a 18X18X24 in. Recommend you cover 3 sides of enclosure to reduce frog's stress.
A baby should be fed daily with gut loaded (veggies & Repashy's Bug Burger) insects that are sized same as distance between frog's eyes. Once grown or when it starts losing interest on daily feedings you can go every other day, etc. Also, dust food with calcium/D3 2X and vitamins 1X weekly on different days with skip day in between. I use Repashy's products but RepCal is also good. Placing food in small glass bowl keeps crickets from wandering and allows to confirm frog is feeding.
Lots of incomplete or erroneous care sheets and videos on net. I trust the one on this site, good luck

!