Stop using the legacy color syntax

Modern CSS color syntax no longer requires the 'A' suffix in RGBA/HSLA or comma separators. The new space-separated syntax uses a forward slash before alpha values (e.g., `rgb(255 0 0 / 0.5)`). This syntax is mandatory for newer color functions like OKLCH, LAB, and the color() function, which support wider color gamuts beyond sRGB. The most practical benefit is relative colors, which allow manipulating existing color variables on-the-fly, such as adding transparency to any color format using syntax like `hsl(from var(--color) h s l / 0.5)`. The legacy comma syntax still works and doesn't require refactoring existing projects.