Improve Dark Theme Support

We did some initial work to improve dark theme support by adding color modifications for the payment element, but there are still outlying issues and testing that needs to be done to improve dark mode support.

I propose that we:

1. Improve our error output for dark themes - note, it may be tricky to detect this on our own so a setting may need be added.
2. Review what is happening with dark mode payment element themes and link. See this image: https://imgur.com/a/6eAqXjP

This was reported via WP.org here: https://wordpress.org/support/topic/input-field-color-customization/#post-18388516