Change themes?

I have two themes downloaded in user/themes, how can I change themes

You name the one you want in system.yaml:

pages:
  theme: materialize

system/config -> system.yaml

pages:
  theme: #your-theme

Thank you for your help. I'm planing to build some themes with Grav cms, looks promising cms with a bright future ahead.

FYI, don't change the setting in system/config/system.yaml, those are intended to be left untouched and serve as system defaults. Change it in user/config/system.yaml instead.

Yes you are right about it.