Global styles: output :root selector for CSS custom properties and remove unused vars

[ramonjd]

Synching:

• Global styles: output :root selector for CSS custom properties gutenberg#42084
• Theme JSON: remove unused vars in layout class gutenberg#59938

Trac ticket: https://core.trac.wordpress.org/ticket/61135

[github-actions]

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Props ramonopoly, isabel_brison.

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[github-actions]

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[tellthemachines]

Thanks for the PR! Tiny comment below, otherwise LGTM!

[tellthemachines]

Committed in r58123.