Hi,
Thank you for the quick reply!
You’re not able to set the margin of the content box using Page Builder, at least not the free version.
I’ve created a new template by making a copy of template-full-width-page-builder.php and called it template-full-width-page-builder2.php
This template is now found when creating a page.
However, with this template the CSS values mrt20 and mrb20 are not used and the content box sticks to the header and footer.
I’ve yet to find a way to solve it.