[6.x] Fix background gradient color in field settings

[heidkaemper]

This PR sets the correct header background color for the field settings stack.

In field settings, the sticky header has a gradient as background. And the colors of said gradient were hard-coded to white and gray-800.

This was not noticeable with the standard theme. However, if a theme had a different color as content-bg, the gradient became visible.

Dark Mode before:

Dark Mode after:

Light Mode before:

Light Mode after:

[jasonvarga]

Thanks! We're actually going to remove that gradient and replace it with the standard/consistent stack header style in a separate PR.