Add global styles

[oandregal]

Part of https://core.trac.wordpress.org/ticket/53175
Follow-up to #1262

To do

• prepare corresponding Gutenberg PR

How to test

A theme without theme.json (use the TwentyTwentyOne theme):

• Load the front. Look for a global-styles-inline-css embedded stylesheet. The expected result is that none should be found.
• Load the post editor. Search for --wp--preset--color. The expected result is that none should be found.

A theme with theme.json support:

• Load the front. Look for a global-styles-inline-css embedded stylesheet. The expected result is that there's a stylesheet that contains the CSS generated by the engine. Depending on the theme's theme.json, it should be something along these lines:

body{
  --wp--preset--color--black: #000000;
  /* more CSS Custom Properties for the presets */
}
.has-black-color{ color: #000000 !important; }
.has-black-background-color{ background-color: #000000 !important; }
/* other classes for every preset value  */

• Load the post editor. Search for --wp--preset--color. The expected result is that there's an embedded stylesheet that contains the CSS Custom Properties.
• Load the post editor. Search for .has-black-color{ ... }. The expected result is that there's an embedded stylesheet that contains the classes for the presets as well as any other block styles defined by the theme via theme.json.

[oandregal]

Need to update the unit tests and prepare the corresponding Gutenberg PR, but the code is ready for review.

[jorgefilipecosta]

In cases where there is link color enabled add_theme_support('experimental-link-color'); we need to output the preset classes, should we have a condition for that here?

[oandregal]

I'm not familiar with prior context on how to deal with this so I'd welcome advice. My thinking is that we should do the following:

• We shouldn't port experimental-link-color to WordPress core. Themes that want that should use enable it via theme.json.
• In the plugin, I'd expect we remove support for experimental-link-color at some point. Probably when the minimum WordPress version for the plugin is 5.8.

Thoughts?

[jorgefilipecosta]

Yes this seems like a plan 👍

[jorgefilipecosta]

Now that this going to be shipped in production would it make sense to have a bigger cache time? cc: @aristath
On another question would it make sense to use add_action( 'switch_theme', ... ); to clean the cache during a theme switch as we do in #1262?

[jorgefilipecosta]

This worked well on my tests and should be ready to merge 👍
I found some issues setting an empty color palette does not work (it defaults to core) but that's a regression setting an empty color palette is a way to disable the color picker. I guess we should look into these issues separately.

[jorgefilipecosta]

Committed in 3d3aa3c this PR can be closed.