Fixes an issue where block style variations that use `ref` values to reference styles elsewhere in theme.json weren't working.
Retrieves reference values when generating styles for block style variations. How? When retrieving variation data, to generate styles from, also retrieve any referenced values.
Reviewed by audrasjb.
Merges [58691] to the 6.6 branch.
Props aaronrobertshaw, andrewserong, ramonopoly.
Fixes#61589.
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