Allow custom CSS for theme.json templates+template parts

[ghost]

What problem does this address?

This is a minor inconvenience, but it means one will have to remember where CSS is allowed and not 😅
Theme.json does not allow CSS in templates and templatet parts, which means one can't do something like

 "customTemplates": [
 	{
 		"name": "blog",
 		"postTypes": [
 			"post"
 		],
 		"title": "Blank",
 		"css": "transition: all 1.1s ease;"
 	},

Instead, one would need to add it to CSS directly under styles and add the selector for the template or template part.

What is your proposed solution?

Allow custom CSS for theme.json templates+template parts.

[ghost]

Hey @carolinan , inspired on your issue: #48439