Hi, there's a few problems I can see which could be causing your frog not to eat.
-Your heat mat is covering too much of the wall
-it's not on a thermostat, it's probably too hot for your frog
-thermometer probe should be directly on the glass where the heat mat is attached - not under the substrate, which will give an incorrect reading.
Another thing, a naturalistic set up is not appropriate for a Horned frog, as they produce far too much waste for this system to work. You also don't need a drainage layer, the frog will appreciate being able to burrow down to moist soil. Just use fake plants, as you'd probably need florescent lighting for real ones to survive. If you do have luck though, leave them in their pots so you can remove them easily as you clean the tank.
What I'd suggest you do is basically start again. Use heat mat (on a thermostat) covering half the wall of one side of the tank. Take the lights off and just use the night lamp, you may also want to provide a lower drop at night time, 75F should be fine. You may not need any heating at all at night but if it drops below 70F in your house, get a thermostat with a night time setting so you can have the heat mat on at night
Jason