Themis doesn't really offer an API, but here is some documentation anyway. You can view the api in your browser or even better, import it in a program called insomnia:
Link: https://glitchcat.github.io/themis-api/
If you happen to test some stuff in the insomnia rest client this is what you should know:
{{ _.base_url }} is just a placeholder for https://themis.housing.rug.nl
{{ _.year_url }} is a placeholder for the year it extracted from the "GET course" call, which is as of writing this equal to /2020-2021
Both of these are configured in the insomnia enviroment, which should automatically get imported.