I cannot see what's inside the collapsed fields while in CSS editor

I cannot see what's inside the collapsed forlders while in CSS editor

Which CSS editor? Your form collapses do open in preview mode, as well in edit mode, and in the Advanced Designer. 

They do not open on my end...

Upon checking, I can see that Section Collapse does not show in the Advanced Form Designer by default. You can try using the CSS code below as a workaround:

.form-section {
overflow: visible !important;
}

Please put that code in the CSS editor. Once you are done, please do not forget to remove that CSS. Here's a screenshot:

Hope that helps. Thank you.

Thanks Jan, it works now!