If you’re hosted by CDCK or Communiteq (formerly DiscourseHosting) then AFAIK it’s 100% managed. You wouldn’t have SSH access or anything like that. Unless you’re on an enterprise plan the plugin options are fixed and everything is tested to be compatible by the respective teams.
You can install themes and theme components on those hosted options without restriction. A test instance is always advisable of course.
I’m not aware of any CD options with CDCK hosting here, maybe one of the team can correct me if I’m wrong. At worst you would be making those changes to the live site in the same way you tested elsewhere.
You can’t install additional plugins on the basic hosted options, they include a number of the most common and well supported by default. You would either need to be on their enterprise tier, or self-host. Plugins need to be rolled into the app.yml and the site rebuilt to incorporate, to disable them you comment out the line and run another rebuild. Themes and theme components can be added and removed without impacting uptime or access to the site by users.