New custom fields(except classic) not accessible from custom theme

I just noticed that new custom fields defined in customer/supplier/inventory items etc cannot be used or manipulated via custom theme. My concern is that, I have been using & still using custom field(classic) defined in customer/supplier/inventory item in manipulating layout of my sales order, invoices etc.
@lubos So what will happen at the time when all the classic custom field will be automatically converted to new custom fields, & old custom fields removed along with custom theme. Will the old documents be affected & will those classic custom fields used in those docs will be missing when viewed after removal of custom themes & classic custom fields?

Can you send some screenshots of the issue in order to make it more clear?

No one can say anything about this issue - all we have been told is that custom themes will be replaced by something but that is all we know at this time.

Same issue here. I can’t use “New custom fields” exactly for this reason.