theme.json schema: fix styles.background definition

[t-hamano]

Follow-up #59354

What?

This PR fixes the following two issues regarding the background property, which can only be defined at the top level of the styles property.

Considered a disallowed property:

Can be unintentionally defined at block/element level:

Why?

The background property is defined as a subproperty of stylesProperties. However, the stylesProperties is also used at the block/element level.

How?

Moved the background property definition directly under the styles property.

Testing Instructions

Create a JSON file like the one below locally and make sure that the background property is only allowed at the top level.

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/fix/theme-json-schema-root-background/schemas/json/theme.json",
	"version": 2,
	"styles": {
	}
}

Screenshots or screencast

4a5a3d161e265e1414cef326e3f031f0.mp4

[github-actions]

Flaky tests detected in 9192442.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/8157326424
📝 Reported issues:

• [Flaky Test] should multi-select block with text selection and a block without text selection #59579 in /test/e2e/specs/editor/various/multi-block-selection.spec.js

[t-hamano]

Unfortunately, if we update the JSON schema correctly, the background section will be missing from the documentation 😅 This is because the script that generates the documentation relies only on stylesProperties and does not consider which level the properties are available.

We may need to make major changes to the logic of the script, or the overall design of the JSON schema, to ensure that it also generates the documentation correctly.

Personally, I think it's better not to have a background property at this point, because if there is a background property in the document, there is a risk of misunderstanding that this property is available at all levels.

[ramonjd]

Agree. Thanks for noticing this detail!

[github-actions]

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Co-authored-by: t-hamano <[email protected]>
Co-authored-by: ramonjd <[email protected]>

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[ramonjd]

The background property is defined as a subproperty of stylesProperties. However, the stylesProperties is also used at the block/element level.

The plan is to allow background a lower levels in the not-too-distant future, but this change makes sense for now and can be reverted later when support is there.

[ramonjd]

Agree. Thanks for noticing this detail!

[t-hamano]

@ramonjd Thanks for the review!