Improve Documentation navlink readability

[Arawn-Davies]

Just a small change that I think would be good, switching between light + dark mode doesn't change the text colour of the navbar, resulting in not great readability in dark mode.

URL of Documentation - What page of documentation are we dealing with?

https://cosmosos.github.io

Actual Behaviour - What is the issue?

I'm guessing it's just in the .css file for the docfx theme but just wondering if there's something else that needs to be done elsewhere... (docfx deployment?)