Add optional styles as props for header containers

[BricePissard]

This optional props styles can be necessary to bypass the {overflow:hidden} attributes set by default in each header styles.

If for instance, a button is placed between the top static header and the scrollable content, it is cropped by this overflow: hidden.

[mcmar]

@BricePissard Can you update the conflicts? I have a repo where I've fixed the README conflicts if you want to use that. I can't update your branch for you and I don't want to open a duplicate pull request.