Templates: make it easier to do template specific style changes

[annezazu]

What problem does this address?

In the seventeenth call for testing for the FSE Outreach Program, the following feedback came in that reflects a likely common action someone might want to take: change the color scheme for a single template.

When I created a template for the “Submission Opportunities” category, I thought I’d change the color scheme for this category from the Styles menu in the sidebar. The Style is not just applied to this single category, but to the whole site. Is that correct? Since I was creating a template for this category, and the call for testing instructions encouraged me to push things a bit, I thought I’d select a whole different color scheme just for this specific category, expecting the style to be only applied to the category and not the whole site.

Here's a quick video showing how it seems at first that changing Styles does the trick, without a clear warning that it'll change the whole site:

change.background.of.template.mov

In many ways, it doesn't seem like it should be this hard to make template specific changes to aspects as basic as background color without changing the entire site.

What is your proposed solution?

Part of this is due to the lack of clarity when saving in multi entity saving (#29388) but the other piece of this is around the visibility of Styles combined with the difficult in making template specific changes. Curious what others think and what can be done as right now it seems like multiple layers need to be considered across global styles, saving changes, etc.

[annezazu]

This use case should be addressed in part by #40318

[jameskoster]

The UI for this could be relatively simple, re-using the 'Settings' and 'Styles' tabs we have in the block inspector. Example:

Settings might not be needed for all templates, but archive templates could have options like "Posts per page".