Add CSS Class for Out-of-Stock Sizes in 'Add to Cart' Form

Currently, the 'Add to Cart' form does not visually distinguish between available and out-of-stock size options. This lack of differentiation can lead to user confusion and frustration, as customers may attempt to select sizes that are unavailable. Enhancing the form to highlight out-of-stock sizes would improve the user experience by providing clear visual cues regarding product availability.

Implement a feature that adds a CSS class to out-of-stock size options within the 'Add to Cart' form. This enhancement will provide users with immediate visual cues regarding product availability, improving the overall shopping experience and reducing potential dissatisfaction.