Add link color and border theme_support

[carolinan]

What?

This PR is intended to be a replacement for the appearance-tools theme support that was merged into core for 6.2.
It adds theme support for link colors and border settings.

I understand this is a lot to read and catch up with.
TLDR:

• The appearance-tools theme support is already enabled in Gutenberg since august 2022.
• It was merged into WP 6.2.
• Bugs were found.
• This PR is intended to add two new, more limited and bug free theme supports
• Next steps would be: Revert the appearance-tools addition to core, merge the two new theme_supports instead.

Why?

1. The appearance-tools includes block gap, which is not supported by the group block in classic themes.
2. By using separate theme supports instead of appearance-tools, we do have more to maintain but it allows theme developers to select which of the individual appearance tools to support.
3. Padding is not included here because it can already be enabled with the custom spacing theme support.
4. Margin is not included because it needs more testing.

I have chosen to add all border settings as one theme support. This is different from how the settings in theme.json works.
In theme.json you enable the color, width, style and radius individually, please comment if you feel these should be separate theme supports.

Closes 41857
Alternative to 47451

Related:
core 57460 Backport: appearance-tools theme_support
core 56487 Bundled themes: opt-in to appearance tools

Testing Instructions

In your WordPress install, make sure that WP_DEBUG_DISPLAY, WP_DEBUG are set to true.
Activate Twenty Twenty-One.
You should see a doing it wrong message about experimental-link-color.
In your code editor, open functions.php in Twenty Twenty-One.
Around line 333, remove
add_theme_support( 'experimental-link-color' );

And add:

add_theme_support( 'link-color' );
add_theme_support( 'border' );

On refresh, the doing it wrong notice should no longer show.

Create a new post.
Add a group block with a paragraph inside. Add a link to the paragraph.
Confirm that the paragraph has a link color setting. Select a custom link color and confirm that it works.
(Palette colors will not work on the front, that is a separate issue)
Add a border to the group, test the different border style, width, and color. (select a custom color with the color picker)
and confirm that it works.

There are known bugs in Twenty Twenty-One that needs to be solved, you do not need to test them as part of this PR,
the main goal is to confirm that the controls are available.

[carolinan]

I considered "being nice" and include the previous name, experimental-link-color in this condition.
What do you think?

[carolinan]

Only one theme in the theme directory uses the appearance-tools theme support.
https://wpdirectory.net/search/01GRB604V9CJQGP77A4H4V5VPK
But we don't have stats for themes outside the directory.

[Mamaduka]

An alternative option could be to allow more granular control via the appearance-tools second argument. If no argument is provided, we use the current behavior - appearanceTools: true.

New usage example:

add_theme_support( 'appearance-tools', array(
    'link-color' => true,
    'border'  => true,
) );

What do you think?

[carolinan]

We could do that, it would be a smaller code change too.
We would only be removing something that is already broken.

My only remaining concern would be that the theme support would have the same name as the theme.json setting, but not include the same features. This would be confusing. I am not sure documentation would be enough, because there would be an expectation that they match.

[Mamaduka]

My only remaining concern would be that the theme support would have the same name as the theme.json setting, but not include the same features.

Adding just add_theme_support( 'appearance-tools' ) in the theme will enable the same features as the theme.json equivalent. This way, we can avoid breaking BC. As I understand, this feature flag was introduced in WP 6.1.

Using arguments will allow us only to enable specific features.

[scruffian]

Next steps would be: Revert the appearance-tools addition to core, merge the two new theme_supports instead.

I'm not sure about this. I think we should still keep an easy way to opt in to everything by default, otherwise themers are going to have to go and update their themes every time we add a new support.

[scruffian]

Only one theme in the theme directory uses the appearance-tools theme support.

What about the appearance tools setting in theme.json? I think a lot of themes use that.

[scruffian]

Using arguments will allow us only to enable specific features.

This seems OK. We could even do a similar thing in theme.json:

    "settings": {
        "appearanceTools": {
            'link-color' => true,
            'border'  => true,
        },    
    },

[carolinan]

There seems to be some confusion here still.
If a theme without theme.json adds the add_theme_support( 'appearance-tools' );
The block gap is enabled but the setting does not work.
So unless that is fixed we should not keep it in core.
We should not let them opt-in to something we know doesn't work.

[carolinan]

The support was never added to WP 6.1, only Gutenberg.
It was added to 6.2 alpha about two weeks ago.