[Migration] Replace Sass with PostCSS: Global Styles and Utils

[seanwuapps]

Objective

Migrate global styles and utilities from Sass to PostCSS to reduce deprecated warnings and simplify our build process.

Files to Migrate

• packages/core/src/global/styles.scss
• packages/core/src/global/scss/_utils.scss
• packages/core/src/global/scss/_typography.scss
• packages/core/src/global/scss/_spacing.scss
• And other global scss files

Tasks

• Set up PostCSS configuration with necessary plugins
  • postcss-nested (for nesting)
  • postcss-custom-properties (for variables)
  • postcss-mixins (to replace Sass mixins)
  • postcss-import (for imports)
• Migrate global variables to CSS custom properties
• Convert Sass mixins to PostCSS mixins
• Update build configuration to process .css files
• Update documentation for styling guidelines

Notes

• Focus on keeping the same functionality while using modern CSS features
• Document any breaking changes
• Update any import statements in components