I’ve moved your post to a new topic, because it has nothing to do with release of theme options.
Your sales invoice was automatically credited because the customer (Walk In Sale, in this case) had a negative balance in Accounts receivable. That should never have happened for a walk-in customer, indicating that you have entered other transactions incorrectly.
Sales invoices have no interaction with cash accounts. They only debit Accounts receivable for the customer specified. A separate Receive money transaction must be recorded when the invoice is paid to credit Accounts receivable and debit a cash account.
I recommend reading and understanding these two Guides:
https://forum.manager.io/t/cash-sales-selling-without-sales-invoices/7321
https://forum.manager.io/t/creating-sales-invoices/7178
And please, stop hijacking topics for unrelated questions.