update styles controller to fetch gutenberg bundled theme.json

[madhusudhand]

What?

Side Editor doesn't load changes from Gutenberg bundled theme.json after this change

This change adds back the method get_theme_item to Gutenberg_REST_Global_Styles_Controller_6_2 so that the local version of the theme.json is fetched when plugin is activated.

Testing Instructions

1. Do changes to bundled theme.json such as updating color palette
2. Open site editor
3. It should have the updated color palette values from bundled theme.json

[noahtallen]

Is this important for inclusion in 14.8.x? E.g. does this fix a severe incompatibility issue? (14.8.x includes the removal of the 6.0 compat code, which has been causing a lot of problems for the release.)

[oandregal]

This fix only affects the site editor, as it's the only place where the REST controller is used, as far as I know.

There are a couple of things I can think of:

• Without this fix, the controller will pull data from the theme.json bundled by core and not Gutenberg, meaning that changes to the Gutenberg's theme.json are ignored.

• It'll also sanitize the data against the schemas defined by core (see VALID_STYLES and VALID_SETTINGS) instead of the schemas defined by Gutenberg: as a consequence, properties defined in Gutenberg but not in the WordPress runtime will be ignored.

For example, comparing Gutenberg with latest WordPress trunk (the same will happen to other props in older WordPress versions), dimensions.minHeight is not show under "Global Styles sidebar > blocks > group > layout" without this fix:

Without the fix	With this PR (expected behavior)

This is definitely not a fatal, so I'm not sure we need a new 14.8.X patch release given how close 14.9. Though it's still an important fix.

[noahtallen]

Ok, good to know. I'll cut a patch release then.