Styles: Level global styles specificity at 0-1-0

[aaronrobertshaw]

To make overriding core styles easier and enable the extension of block style variations as a means for section styling, it is necessary to limit the specificity of global styles CSS output.

Further context can be found in: WordPress/gutenberg#57537

Syncs changes from:

• Try reducing specificity of global styles selectors only. gutenberg#60106
• Try reducing specificity of layout style selectors gutenberg#60228
• Reduce specificity of global styles body margin reset rule. gutenberg#61340
• Styles: try wrapping with :root to fix reset styles gutenberg#61638

Trac ticket: https://core.trac.wordpress.org/ticket/61165

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[aaronrobertshaw]

@tellthemachines and @andrewserong, if you get the chance, it would be great if you could confirm this backport covers all it should from previous zero-specificity work and the closed PR: #6522

[tellthemachines]

Code LGTM and it's testing correctly on my local. Just need to add @since 6.6.0 (for functions) and @ticket [ticket number] (for tests) and should be good to go!

[andrewserong]

Thanks for the ping, this is all testing nicely for me, too!

Just need to add @SInCE 6.6.0 (for functions) and @ticket [ticket number] (for tests) and should be good to go!

+1 🚀

[aaronrobertshaw]

Thanks for the reviews and nudge to get the since/ticket comments in place. That's been done now.

I've also given this a rebase and updated the new background image test for theme.json.

[github-actions]

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[tellthemachines]

Thanks for the update! This is looking good to me now 🎉

[tellthemachines]

Committed in r58241.