Btcpayserver: Feature Request: Add CSS customization to Pull Payments (& concurrently Refunds)

[Zaxounette]

Is your feature request related to a problem? Please describe.
Currently, it's possible to customize the public view of Payment Requests using either pre-made bootstrap files, or inserting custom CSS code to be rendered on the Payment Request page.

Describe the solution you'd like
Such customization is currently missing from the Pull Payment feature.
Additionally, It could be nice to also have it included while issuing a Refund as well, not only a direct Pull Payment creation.

Additional context
CSS Customization in the Payment Request feature currently missing in Pull Payments (Screenshot pulled from the Documentation):

This feature request follows a discussion with @dennisreimann earlier this week.

[bolatovumar]

@dennisreimann are you looking into this or should I try looking into it?

[dennisreimann]

@bolatovumar go ahead :)

[pavlenex]

Added in #2113