Lower the priority of the gallery gap css so it loads after the block layout css

[glendaviesnz]

What?

Fixes the order that the Gallery gap CSS is enqueued so it is not overwritten by layout CSS in non block themes

Why?

Currently for non-block themes the Gallery block gap CSS is being loading prior to the layout CSS, so the gap value is overwritten by the default 0.5em value

How?

Set the action priority lower in the Gallery render method. This issue is also fixed by #41015, but this smaller change may be easier to backport to 6.0.1?

Testing Instructions

• In a non-block theme, add a custom CSS var or --wp--style--gallery-gap-default: 37px;
• Add a Gallery and check in the frontend that the gap is set to 37px
• Also try and block theme and make sure gap still works as expected

Screenshots

Before:

After:

[andrewserong]

Good idea @glendaviesnz, this looks like a simple way to fix the issue in the short-term to me!

✅ The gallery block spacing feature still appears to work correctly in the post and site editors with a block theme.
✅ In a classic theme, adding in a rule like :root { --wp--style--gallery-gap-default: 46px; } in the frontend and editor CSS allowed for overriding the gallery gap's default 0.5em on the site frontend and within the editor.

Before	After

LGTM!

[ramonjd]

What @andrewserong said 😄

Tested in block/classic themes the latter by adding --wp--style--gallery-gap-default + value to the :root selector in frontend and editor stylesheets.

Tested on 2021, 2020 and 2022 WordPress themes.

[adamziel]

I just cherry-picked this PR to the wp/6.0 branch to get it included in the next release: 10f347a

[adamziel]

@glendaviesnz this PR contains PHP changes that need a matching PR in the https://github.com/WordPress/wordpress-develop repository to be included in WordPress 6.0.1. Would you please create such a PR? CC @SergeyBiryukov

[glendaviesnz]

@adamziel WordPress/wordpress-develop#2839 - I assume we branch of 6.0 for this, but let me know if this was the wrong assumption and I will redo the PR

[adamziel]

@glendaviesnz I just commented on the other PR, we're good!