Provide Material You palette as CSS variables

**Is your feature request related to a problem? Please describe.**

Currently theming, in particular separate colors of a theme, is limited to separate theme options, and it has to be manually selected. With MD3 variables this can be avoided as the theme itself can use these variables for coloring.

**Describe the solution you'd like**

When available, Material You colors should be added as CSS variables, something akin to this:
```css
:root {
--system-accent1-1: ...;
--system-accent3-100: ...;
--system-neutral1-900: ...;
/* and so forth, you get the gist */
}
```

I would attempt to make the PR myself, but I do not have an Android device that I can easily toy with, nor do I have much experience with Android Studio and alike.

**Describe alternatives you've considered, if any**

No alternatives are currently possible (to my knowledge)

**Additional context**

There are already existing themes that try to emulate the design of MD3, but again, having to have separate themes for the various colors, with those themes and this feature added, you'd effectively be able to have a proper Material You and MD3 Home Assistant app.