Bokeh server and templates

How can I use a HTML template when I start Bokeh server as library within Tornado (Running a Bokeh server — Bokeh 2.4.2 Documentation)?

Concretely I use this (GitHub - snowch/bokeh-cloud-foundry: Example for running bokeh server as a Cloud Foundry application) example.

I embedded an index.html in the templates folder as in the examples on GitHub, but does not affect the web page in any way. Do I have to reference it?