Thank you all for your input!

Nagol, your idea to test was a great one! The 150 watt ceramic heat emitter gets very hot, so I have suspended the bulb about 6 inches above the screen of the vivarium and was able to get the night time temp to about 74 and during the day at about 85. The frogs were more active last night, so I think they're happy to have more warmth. But, the downside to this heating method is the humidity drops fast. So I'll need to be more diligent with misting, which is fine because I work from home.

Kristen and mim, I do have and use a styrofoam background that came with the enclosure, but I think insulating the 2 sides of the tank is also a great idea. I am going to get my husband to fit cardboard to the sides, so that will also help I'm sure. Thanks for your input.

Carlos, I do have a thermostat/controller that I've hooked the lamp into, so if the tank gets above the set temperature it turns off. I wouldn't recommend anyone use one of these ceramic heat emitters without one as they do get so hot, so it's definitely a very important safety feature to have in conjunction with the bulb.

Sajane, I did rest a dome directly on the screen top previously when I was using a 50 watt red incandescent and never had a problem... But my frogs hate the light at night, so didn't use it unless I felt it was absolutely necessary. However, I would never rest a dome with a ceramic heat emitter in it directly on the screen top, because like I said it does get really hot and could possibly burn your frogs.... They make stands you can buy that suspend the bulb at the height you want above the vivarium, which is what I am now using and i'm very happy I invested in it.

Thank you everyone for your help!!!