Are theme parks really not a thing in central Europe?
I think that entertainment is seing differently perhaps where you come from compared to Europe in general. As in the USA I have seen a lot of adults in amusement parks, it is not so much the case in europe in general. People would prefer to travel to cultural places or to go to public baths in eastern europe for instance.
From Prague by public transport, what would be the closest proper amusement park? With 'proper' meaning attractions are not just for kids and it's not an aqua park.
Firs of all what do you mean by public transport ? Buses and subway ? If so you are naworring it down a lot... For amusement parks people are willing to travel longer in europe than they would in the USA due to the lack of amusement parks in general.
The "Best" theme parks not for Kids only would be :
Disneyland Paris : You can take a train that would bring you direclty at the doors of the park from Pragues and if you book your tickets early enough It would be cheaper.
Europapark, Germany : It is considered as a great park for young adults as the rides are not solely for Kids, but it is a little bit more complicated to get access to it from prague.
Port Aventura, Spain : A bit remote for you but also a great park for young adults. In order to get there from prague you would need more time and a flight is necessary.
Phantasialand, Germany : It is a very different amusement park but It is worth the visit. Same as for the other german park, you would need to take a train or to book a blablacar ride.
Tivoli Gardens, Denmark : it is the oldest amusement park in the modern europe and is quite cute. I have checked and you can find flights for less than 35€.
Gardaland, Italy : It is supposed to be a high sensation park. Also you can access it by train or plane or blablacar.
Googling around I have been able to determine that there are no amusement parks in the Czech Republic to travel to
Maybe the "Proper amusement park" is not around you, and you can choose either to travel a bit or to go to fun fairs around you which have the advantage of being pretty much everywhere but you won't have access to that much adult rides.