Add custom properties for border radius

[unscripted]

Changes proposed in this pull request

This PR adds a set of custom border radius properties to the theme.json settings. These properties provide a standardized way to apply consistent border radius values throughout the theme.

Resolves #73.

The problem this solves

Currently, border radius values are being applied inconsistently across different components and blocks. This makes it difficult to maintain a cohesive design system and requires manual updates when design requirements change. By introducing these custom properties, we can:

• Ensure consistent border radius values across the entire theme.
• Make it easier to update border radius values globally.
• Transfer patterns between projects.
• Integrate with the block editor's border radius controls.

Proposed documentation

Kindling provides a set of predefined border radius values that can be used when setting global styles. These values are defined in the theme's theme.json file under settings.custom.border.

Available border radius values

Property	Value	Description
rounded-full	calc(infinity * 1px)	Creates a fully rounded border (pill shape)
rounded-large	24px	Large border radius for prominent elements
rounded-medium	16px	Medium border radius for standard elements
rounded-small	8px	Small border radius for subtle elements

Additional resources

• The naming schema of CSS Custom Properties
• CSS Custom Properties (Variables) MDN Documentation

Pre-submit checklist

As the author of this pull request, I verify that:

• I have set the target branch to main.
• I have detailed the purpose of this Pull Request in a non-technical way.