What's the point of a desk surface you can't write on? I see what it is and the message but in the end function matters. I agree the chair has to go. I would look for a light carpet to dispel the darkness a bit.

It will be tricky about what size rug you select and how you place it. Normally I would NOT place the rug under the desk chair. The location get too much wear and tear and shows signs of soiling immediately.

Select a light colored contemporary pattern rug for this location.

Which chair are you suggesting we ignore - the one behind the desk or the upholstered one to the right?

If you’re keeping the chair on the right, I agree that you should find a light colored rug to blend with it. And also agree that the rug have a pattern to disguise wear.