The templates are PNG files and are located in
/usr/share/remarkable/templates/ you add new templates by simply copying additional files to this folder using SSH Access.
The templates are in the exact size of 1404×1872 pixels. The Menu Bar, if not closed, takes exactly 120 pixels on the top and on the left side.
There was a tip posted on Reddit about changing the default template that is used for new notebooks.
Blank.png is set to be used as the default template when creating a new notebook.
To have another template make a backup of the original
Blank.png (eg. rename to
Original-Blank.png) and just to be sure make a copy on your computer in case of an update overwrite.
Then rename(or copy) the template you want to use as the default to
Blank.png and you have a new default template!