This issue was identified by Microsoft during an internal audit for accessibility.
Currently, when a Windows user has enabled High Contrast mode, there is no visual indicator that a particular item in the course nav is the page they are currently on.
In the image above, I would expect to be able to tell that I am on the Notes page in the nav.
Elsewhere in the platform, we bold the text in links and buttons for the current page. I suggest the same fix here. I'm pretty sure you can fix it in this class definition here: