Themes: Utilize the @property rule to better describe global CSS Custom Properties

[simeonoff]

We generate a bunch of CSS Variables that we include with each mixin, some of which are very specific to how they should be used/configured. This issue should be expanded with specifics, but the general idea is to better type some of the CSS variables using the @Property rule so that we:

1. Limit the possible values that can be passed to a CSS variable we provide.
2. Reduce the amount of :root declarations with better defaults.
3. Provide better DX.