Custom themes are now an unsupported, obsolete feature. The developer does not recommend any new custom themes be created.
What do you mean by “previous Due Report?” That terminology has no meaning in Manager. If you are referring to a previous balance for the customer, that has been discussed many, many times in the forum. The variable does not exist, so even if you were to write a new custom theme, you could not include it. A recent discussion was here: Previous customer balance in current invoice.