Thank you for posting about customizing your statement form. I'll share information about it.
It depends on the level of customization you want. Basically, we can't customize a statement form as it is. However, when you customize the Standard template, for example, you change the style, the font, the color, or anything in the design, it will also reflect on the statement form. Then, since it's the Standard template, the changes will also reflect on other non-customizable forms such as a credit memo, a refund receipt, and a purchase order.
Now, it's up to you to decide if you want to customize your statement by editing the Standard template. I've already shared the consequences above. If you'd like, you can use the QBO test drive before making changes in your actual QBO company.