Problem with fields in invoice

I am using before last update fields(0).text and fields(1).text to get date in custom invoice template and now I can’t

custom themes are responsibility of the users.
Manager is continuously evolving and due to this the variables would change.

yes of course you are right. Have this fields change name and can’t be selected as fields and number?

that would depend on what version you were using previously.
it is always better to loop through the relevant lines of code than referring the variables directly.
if you have programming skills you can adapt to the changes easily. if not, it would be better to hire a local programmer to do it for you.