Display custom fields below its source on Plain theme

@lubos I would like to request that the default plain theme displays the custom fields below its source on transaction forms. this would eliminate the need of a custom theme for the same.

for example, the custom fields created for Customers are displayed along with the other custom fields on a Sales Invoice.

previously, the Address field was utilized to add details of customers or suppliers which needed to be displayed on transaction forms.
now, few of these details need their own dedicated custom fields to fetch data for report transformations. but the issue is that these custom fields are currently displayed in an irrelevant place on transaction forms.