Read about custom themes here: [16.11.24] Added ability to create custom themes - #59 by Tut.
The switch from Invoice to Tax Invoice is accomplished automatically when using a tax code for a country where this is known to be a requirement. If by “Performa Invoice (Simple Bill)” you mean a “Pro Forma Invoice,” there is no such form in Manager. You can rename a sales quote as a pro forma invoice, but such a form has no financial impact. So it cannot be used for recording actual sales.
On the other hand, if you want a simpler record of receiving money, just use a Receive Money transaction.