How to apply custom styles on payment processing tool products?

Furthermore, I know it is possible to apply custom formats to such a payment form, I just don't know how, and by that I mean to change the look of the checkboxes and modify the appearance of the sub-labels and the text inside. Could you help me with this please.

I would suggest checking out this guide: How to apply custom CSS to a particular field

It is also linking to another guide: Form Designer Tutorial: Let's create fantastic forms! which I would also suggest checking out.

If you need any specific wishes but are still not quite able to set it up right or not sure how to do it, just let us know and we would be happy to assist.