The has-z class is only added when certain settings (can’t remember which) are enabled so it is better to switch it off than override it for every stack on the page.
Tip of the week: if you do want to override something then give It a custom class and just override the one that is problematical rather than every one on the page which can mess up other layouts.
Also - if you get into a situation like that then it is almost certain that the layout is wrong or unnecessarily nested. There should be no need at all to have Chroma inside anything else for a simple header setup like that. Try and keep things simple and don’t overcomplicate to find a solution - simplify instead.
@Geoff not at all - just make sure your CSS is scoped using a custom class.
Anyway, I am back at the computer now and so I have found the setting. @steveb It is Advanced>Background in front of page cover
that you need to turn off
That setting can always be off if you are using Foundry. It is only there to stop Blueprint backgrounds going behind the Foundation cover image which is actually a div of its own (vegas.js slideshow) and hides the background and overlays of CSS backgrounds.