refactor: update dark mode external link colour

[alexbarnsley]

Summary

https://app.clickup.com/t/863gtt41u

Checklist

• I checked my UI changes against the design and there are no notable differences
• I checked my UI changes for any responsiveness issues
• I checked my UI changes in light AND dark mode
• I checked my (code) changes for obvious issues, debug statements and commented code
• I opened a corresponding card on Clickup for any remaining TODOs in my code
• I added a short description on how to test this PR (if necessary)
• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged