DITA Element Definitions

Hello, I am new to DITA and I want to start working with some DITA topics. I started with the following order: 

File > New > DITA > Topic 

Now FM loads a list of possible elements. So my question is, where is the corresponding FM file where the elements are defined? I want to add some elements and modify the fonts, therefore I searched but couldn't find the correct file. 

I started searching as described in the official FM help:
... \ AdobeFrameMaker2019\Structure\xml\DITA_1.2\app

Thanks very much for your help, Hannah