I only see email templates. Where is the means to be able to change the appearance of invoices?
If appearance by you mean format/content placement of invoice, its not dynamic from GUI side
Looking through code of yours can be challenging due to the lack of comments, so it makes it a bit harder for those wanting to contribute. A lot more time is used up trying to figure out what is going on in the code to make changes for those who are not familiar with it.