refactor(blocks): theme observer singleton

[fundon]

Closes: BS-975

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[fundon]

• #7736 👈
• #7746
• master

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Merge activity

• Jul 29, 9:43 AM EDT: Saul-Mirone added this pull request to the Graphite merge queue.
• Jul 29, 9:59 AM EDT: The Graphite merge queue couldn't merge this PR because it had conflicts with the trunk branch.
• Jul 30, 10:17 PM EDT: The merge label 'merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jul 30, 10:21 PM EDT: fundon added this pull request to the Graphite merge queue.
• Jul 30, 10:40 PM EDT: fundon merged this pull request with the Graphite merge queue.