I must be doing something wrong.
I am on macOS Catalina and Sigil 1.9.20.
I have placed user-template2.xhtml and user-template2.css in User > Library > Application Support > sigil-ebook > sigil. Then restart Sigil.
Right-clicking in Book Browser to Add Blank HTML file and Add Blank Stylesheet simply create generic blank files, not my templates.
Help, please??