Would be fine to upload and use in Preview mode my own CSS.
We have some changes (font - family/size etc.) done with skin builder. The effect if I import ohne of JSONs to form builder and click preview many of the labels and fields are moved so it´s hard to use form builder.
The one and only real possibily is to “draw” the form with form builder, export, open in our application, make some position correctors here manually and import it again to finalize the form.