On Venus the surface is around 467 degrees Celsius. At the top of the atmosphere it is much cooler, and at around 60 km above the surface the temperature is around 40 to 50 degrees Celsius. This is a large difference, and I think it could be used to generate power. Could one build a structure to harvest energy from this temperature difference?
100% YES It s called an updraft tower.
You can actually even do that on earth in the Deserts (solar updraft towers).
On Venus you wouldn't even need the "solar" part. Just build a tall updraft tower.
The question is would it make sense? By the time we want to colonize Venus, we would probably have shifted to a hydrogen economy (cold fusion based) so it would probably by orders of magnitude cheaper to just bring your cold fusion generation with you instead of building updraft towers on Venus given that a few kg of Helium3 would be enough to power a colony for centuries.
Yes you can.
As a simple example, you could create a giant tube and put a Fan in it attached to a generator. As the hot air moves upwards (hot air is less dense and will go up) it moves the fan which spins and generates power. A more advance version would be similar to our coal plants which use the coal to heat water into steam, then that pushes through a giant set of fans which form a generator. The water is then cooled by the atmosphere and reused.
You can try this in real life. Get a tube, get a pinwheel and place it over the top opening. Place a candle under the bottom opening. Over time, you should see the pinwheel spin slowly as the hot air pushes past it (If you don't accidentally set it on fire that is).
As @AlexP mentions, the real question is if its worthwhile....
You might want to instead use the wind speed gradient. If you have a balloon habitat about 50 km up where the temperature is comfortable your habitat will be moving with the wind at a few 100 km/hr (relative to the ground), while the atmosphere lower down will be moving much slower. Lower a wind powered electric generator on a cable to where the relative wind is at least some 10's of km/hr & use that for powering your habitat. https://en.wikipedia.org/wiki/Atmosphere_of_Venus