Theme JSON: replace top-level background style objects on merge

[ramonjd]

A PR that syncs the Gutenberg PR:

• Theme JSON: replace top-level background style objects on merge gutenberg#66656

This PR fixes an omission in the theme json merge logic where top-level background image objects are not replaced, rather they are merged. Blocks are already catered for via ::get_block_nodes().

To test, run npm run test:php -- --filter=Tests_Theme_wpThemeJson

Trac ticket: https://core.trac.wordpress.org/ticket/62328

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[andrewserong]

As in the Gutenberg PR (WordPress/gutenberg#66656), the logic looks good to me, and this is smoke testing well in theme.json and the site editor. I haven't run into any issues as a result of replacing rather than merging the backgroundImage property.

LGTM! 🚀

[ramonjd]

Committed in r59335 / b1b5761