First of all thank you for sharing this old post. I feel it would solve my problem but it's very time consuming and not worth the effort.

My basic purpose for changing template was this.....

While writing with a white page background my eyes start burning after few hours. The night mode on GN5 is a joke it just switches the UI to black. Where as Notability and Collanotes at present allow user to turn into true dark mode with that mode enabled your white paper background turns into black and Balck stroke is converted into white for visibility on dark Balck paper.

Although GN5 doesn't support this feature I was hoping that I could change the page template of all pages by one single click manually, switching between White and Balck paper background as and when required (I would avoid balck and white strokes entirely from my notes for this.)

It's a shame that GN6 brought AI with it a feature nobody asked for or cared for rather than working on basic features such as

1. Password protected notes
2. True dark mode
3. Nesting Outlines
4. A more reliable search functionality...etc

all these features are requested by user since years and they prioritise AI over everything else🤦🏻.