It might be important to someone, but it doesn't affect me thankfully!
I also tried on my desktop with GTX 1080s, and high contrast mode does work as expected, including inverting the colours of the GUI, so it must just be related to my laptop (Dell Precision M4800).
I am using the Metashape classic theme, see attached (this is from my desktop where it works).
In Dark or Light modes there was no effect to the display on my desktop, as you discovered.
On my laptop there was also no affect to the display, with the different themes, except that it became 'unresponsive'. However, switching out of high contrast mode the display did update again, showing that my clicking had had some effect (the trackball was rotated for example).