Separate structural CSS from theming

[kAlvaro]

It would ease integrations a lot if CSS only contained the bare minimum structural styles to make the widget work (position, display...) and additional theming (margin, padding, font-size, colours, shadows...) came separated.

Current styles are very difficult to override without breaking the library functionality.

Thank you!

[arendjr]

I'm open to this suggestion, but I don't have a personal use case for this. Also I think this would introduce a sort of "CSS contract" where changes to certain styles could be considered breaking changes according to semver, which I'm afraid might create extra overhead in the release process. But if someone can provide a good PR, I'm willing to consider it.