[Feature Request] Support for light/dark theme versions

[mdentremont]

Some wallets like Apple Pay support the ability to show a different icon depending on the theme of the page it is displayed on.

It would be nice if this could be supported here as well - essentially working like:

• If only one button, assume it is used either way
• If a dark & light theme icon are provided, show dark on dark checkouts, and light on light themed checkouts