Add the absolute minimum to get started

[bobbingwide]

1. style.css
2. index.php
3. block-templates/index.html
4. experimental-theme.json

File	What if missing?	What if empty?
style.css	Theme can't be found / activated	?
index.php	Template is missing. Standalone themes need to have a index.php template file. Child themes need to have a Template header in the style.css stylesheet.	Nothing shown, but it's not a really a WSOD
index.html	The output from index.php	Empty template: Index
experimental-theme.json	WSOD on Edit site	WSOD on Edit site
functions.php	style.css is not enqueued	ditto

[bobbingwide]

Note: If you have an empty index.html, causing 'Empty template: Index' then try to invoke Edit site you get a WSOD.

The same occurs when it only contains, a Classic block - no HTML comments

This is index.html

or when it's a custom HTML block.

<!-- wp:html -->
This is index.html
<!-- /wp:html -->

[bobbingwide]

Hmm. not making much progress here. Functions.php is now pretty much like I had it in Fizzie and SB.
If I try to click on Edit site or Site Editor beta ( https://s.b/wp55/thisis/wp-admin/admin.php?page=gutenberg-edit-site ) then I get the WSOD.
If I first Edit the post/page then choose Status & visibility >Template (Edit), then I can edit the template.

It would appear that the system's expecting to find something important in experimental-theme.json.
I created a dummy file.

{ 
}

It worked. I've updated the table in the main issue comment.

[bobbingwide]

Note: If you have an empty index.html, causing 'Empty template: Index' then try to invoke Edit site you get a WSOD.

This was a red herring. It was the missing experimental-theme.json file that caused the problem.
An empty index.html does work.

[bobbingwide]

It would appear that the system's expecting to find something important in experimental-theme.json.
I created a dummy file.

I actually committed an empty file.

Conclusion:

• style.css and index.php are needed in order to allow the theme to be activated.
• But style.css doesn't need to be enqueued.
• so there's no need for functions.php
• block-templates\index.html and experimental-theme.json must exist
• but both can be empty.

Now that's what I call a small theme. OK, it's only a starter theme.