fix: QR background colour in dark theme

[cvarley100]

Summary

It is not possible to scan the QR code with an Android device when the desktop app is in dark mode. This PR adds a white background.

Changelog

- Adjust QR presentation in dark mode

Testing

Platforms

Tested on Mac OS, scanning with both iOS and Android.

• Desktop
  • [x] MacOS
  • [ ] Linux
  • [ ] Windows
• Mobile
  • [ ] iOS
  • [ ] Android

Instructions

Scan the receive address QR code.

Checklist

• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or modified tests that prove my changes work as intended
• [ ] I have verified that new and existing unit tests pass locally with my changes
• [x] I have verified that my latest changes pass CI workflows for testing and linting
• [ ] I have made corresponding changes to the documentation