How can I create new templates?

the menu entry “change template” is always grey. How can I create new templates and configure the engine in a way that these templates can be attached to my content?

You have to create new page blueprint in /site/blueprints/pages. The you have to set which templates/blueprints can be used for subpages in your pages sections.

I recommend you read the annotations in the starterkit sourcecode and follow the links to the corresponding sections in the docs.

Going through the first few chapters in the guide to get a basic understanding of how it all works is also not a bad idea.