Simplify Dark Scheme Variables

[ventaur]

I would like to propose simplifying the application of variables for the Dark Scheme. Instead of having to define variables twice due to the media query for the user's preference and the case for forced dark scheme, we could use the light-dark color function. For forced elements, we could set color-scheme based on the data-theme attribute.

Browser support is solid at this point, unless folks are stuck on older versions (pre-2024). I've used it on a couple of projects with no issues.