[Global Styles]: Add REST API endpoint to fetch variations

[ntsekouras]

Part of: #35619

In this PR the REST API endpoint is being extracted from the main above PR.

As noted in the original PR about WP_REST_Global_Styles_Controller:

This is just the exact same class that is in Core, the previous version in the Gutenberg plugin was missing some changes that landed directly on Core WordPress/wordpress-develop@9f0d8fd

[ntsekouras]

I think we should rename get_theme_items to get_theme_variations or change the endpoint to be theme-items. This all comes down to whether we want to support more things from this endpoint other than fetching the variations.

[Mamaduka]

We will merge this into WP_REST_Global_Styles_Controller in the future, right? In that case using get_theme_variations or get_variation_items make sense.

[ntsekouras]

In order to properly test this, I can see three options:

1. Create the styles folder in the theme and a variation (current implementation, though it seems to have permission issues on the repo that needs to be checked.
2. Add a new dependency of vfsStream as explained here
3. Add variations to empty theme in the repo.

I'm not really familiar with php tests, so I could use some feedback.

[Mamaduka]

Add variations to empty theme in the repo.

It would be helpful for e2e tests as well. We also have two other themes, especially for PHPUnit tests; you can also use them.

[ntsekouras]

Add variations to empty theme in the repo.

It would be helpful for e2e tests as well.

Actually this sounds better, yes. We will have e2e tests later on.

[ntsekouras]

This in core is $theme->get_raw_data()['styles']; assuming that every theme.json has styles property. We'll need to backport to core. What I'm wondering though should we set this to empty array or not set at all? @oandregal , @jorgefilipecosta

[ntsekouras]

Core patch here: WordPress/wordpress-develop#2224

[youknowriad]

Nit: I wonder if we should try to make the variation more "meaningful". For instance add a "dark" variation to the empty theme.

[ntsekouras]

I can do it an a follow up to unblock the remaining functionality from landing. I'd appreciate if some theme dev could provide a minimal dark theme.json for empty theme. --cc @kjellr 🙏

[kjellr]

Hmm. So this would be just for the emptytheme version bundled here with Gutenberg, right? For the canonical version over in theme-experiments, I don't think adding a style makes sense since it's positioned primarily as an ultra-minimal starting point.

It doesn't even specify a color palette, so making a dark mode version of it is a bit unexpected. 😄

[youknowriad]

LGTM 👍