My lighting heats my viv for the darts. I adjust mine by the height of lights and amount of venting I provide at the top.
I have glass over a screen on top and then slide the glass to expose more screen as needed.
I also don't allow the tank to get near 85 since above 85 for extended time can kill the darts,

I use heat pads on the sides of my tree frog and crested gecko tanks since the heat has to work through the substrate to heat the air if installed under the tank, where on one side creates heat warmer air on that side and cooler temp on the opposite.