Add a colored variant for Container style

[wiiznokes]

This is useful for debugging

[dtzxporter]

Instead of adding a new variant, it may make more sense to implement From for the style like some of the other widgets do so you can just have a colored container that way without adding another branch here.

[hecrj]

Made some changes so that the following works:

container.style(container::background(color))

Thanks!