WooPay logo doesn't read well on the order received page when the theme has a dark background.

[brettshumaker]

Describe the bug

We're displaying the WooPay logo on the order received page to highlight that the order was completed via WooPay. When the theme has a dark background, however, the "PAY" part of the WooPay logo all but disappears leaving a lonely "Woo" bubble.

To Reproduce

1. Set the site's theme to one with a dark background (I used Twenty Twenty-Three with a dark background style).
2. Complete an order via WooPay.
3. See the hard-to-read WooPay logo on the order received page.

Additional context

I think there's an opportunity here to utilize the UPE Appearance Theme transients used elsewhere to detect when the site has a dark background and display the white version of the logo when appropriate.