Hi Karin,
Glad to hear your frig has shown improvement :-)

I would pick up humidity and temp gauges at a local pet store. When selecting the gauges, compare their readings and if several are reading similar select one of those since they are not the most accurate things.

You want the humidity around 60 - 70 percent.

Since your using fake plants you will have to mist more often.
If you put a full screen top on, you can cover it partially with a piece of glass or similar to cut back on ventilation and help retain humidity.

I'd recommend reading the care sheet for your frog found here: Frog Forum - White's Tree Frog Care - Litoria caerulea