I wouldn't worry too much about that. It is actually very preferable to have a variety of temperatures your frogs can choose from. The only thing I would be careful about is if the temperature drops too low at night in that bottom portion. If you're really concerned about it, try sticking a small heat pad on the back wall? Honestly, introduce the darts and see what happens. If they're hanging out in the mid and top portion of the tank then you know the base it too cold and you should provide some more warmth there.

It looks awesome by the way!
Alex