Fix Sass deprecation warning for darken() function

• Replace deprecated darken() function with modern color.scale()
• Add required @use "sass:color" import
• Maintains same functionality (darkening color by 10%)
• Resolves deprecation warning in Sass 1.78.0+

Fixes #3477974

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

Closes #3477974