Most dart frogs are good at "room temperature". That is, high 60s to mid 70s. Some can go warmer, although most don't need or appreciate it. Some can go cooler as well. I don't use heat mats on any of my tanks. If you were going to the recommendation is to place them on the back of the tank. This is because heating mats shouldn't be below water (if that makes sense). In this case just use the sticky ones that will adhere to the back. However, I don't think they're really necessary. If heating is an issue, you can put lights on the tank. That should heat it up fairly well during the day. Then just let the tank cool naturally over night.

~Alex