Support theme variants / flavors

[LasseRosenow]

It would be nice, if a theme could provide multiple flavors like a blue, red or green version etc.

Right now if you create a theme but want to give your users the ability to choose between multiple pre defined primary colors, you would have to publish the same theme multiple times with the only difference being the primary color.

Just like you had to publish a dark and light version in the past, which now is supported by home assistant to be provided within the same theme, it would be nice to also provide custom variants of a theme just withing the theme itself.