yes you can add it to any theme.
In documentation code is for reference.
You have to use Components in backend to add to any layout or page.
Steps to add:
Click on CMS menu
Open any page where you want to add contact form
click on Components on left menu
drag Simple Contact Component on page where you want to add it
to change html or classes:
click on Component where you add form in your html
a fork icon will appear on top right corner of editor, clicking on it will reveal the html
Hope that helps