I see that “Show custom field on printed documents” in bank account doesn’t show off in the Bank Account Summary. Where is supposed to be printed?
Custom fields for bank accounts are not currently intended to be printed on reports like the Bank Account Summary. No reports except statements include custom fields. (Reports other than statements are coded differently and do not use themes.) The reason the option appears when defining a custom field for a bank account is that the same entry form, tagged as custom-field-form, is used for all regular (non-line) custom fields.