Build bulletproof color systems

The CSS `contrast-color` function is now baseline newly available across all browsers, automatically selecting black or white text based on a background color. This tutorial demonstrates building a flexible button color system using CSS custom properties with a 'private property' pattern (underscore-prefixed variables) for clean component theming. The approach combines `contrast-color` with `color-mix` and `light-dark()` to create buttons that automatically adapt text color and hover states across light and dark themes without manually declaring text colors.