Fix global button styles for colors and borders

[donnapep]

Resolves #

Proposed Changes

Previously, we were applying colours to the .wp-block-button element, which caused issues with colours not working when set through Global Styles. To address this, colour styles have been moved to .wp-block-button__link to match what core does for the Buttons block.

Testing Instructions

1. Activate any theme besides Course (learning-mode-compat.scss isn't loaded for Course theme).

New/Updated Hooks

Deprecated Code

Pre-Merge Checklist

• [x] PR title and description contain sufficient detail and accurately describe the changes
• [ ] Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• [ ] All strings are translatable (without concatenation, handles plurals)
• [ ] Follows our naming conventions (P6rkRX-4oA-p2)
• [ ] Hooks (p6rkRX-1uS-p2) and functions are documented
• [ ] New UIs are responsive and use a mobile-first approach
• [ ] Code is tested on the minimum supported PHP and WordPress versions

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Test the previous changes of this PR with WordPress Playground.