Twenty Twenty-One: Add border support

[poena]

Part of https://core.trac.wordpress.org/ticket/63072

Start by adding border support to the theme by adding add_theme_support( 'border' ); to the theme's setup function.

It is expected to need to adjust the SCSS as well as the dark mode feature, for the border to work correctly.

Twenty Twenty-One uses SASS. This means that changes to stylesheets need to be made using the .scss files, that then needs to be built to be converted to .css.

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How to update the theme css files

This step is needed for both providing a patch and testing a patch:

After installing the theme you need to open the theme's folder in your terminal and run two commands:

npm install
npm run build

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What to test

This is an enhancement, only test the pull requests.

The task is to confirm that all border settings work on all blocks that supports borders.

Which blocks that support borders changes with each WordPress release.
For WordPress 6.7, the list of blocks that supports borders can be found in this blog post:

​https://make.wordpress.org/core/2024/10/17/roster-of-design-tools-per-block-wordpress-6-6-edition-2/

Not all blocks have support for both color, style, width, and radius, and that is OK: Only test the available settings.

The editor and front should match.
Test all alignments and width options with the border.
test all block variations, for example the grid, stack and rows, and the rounded image variation.
Test both palette colors and custom colors.
If a block has an option to enable a lightbox or a link (like the image block), then the border needs to be tested with and without this option enabled.

How to add a test report

There are many core blocks to test. It is OK to only test a few at the time.
Please test without plugins active (other than Beta tester and Test reports).

In your test report, please include:

• The name of the core block you tested.
• The actual result: Did the color options work?