@ryans @JTL More than two and half years later..., I switched to SSH secure shell (TM) after realizing the problem with PuTTY. The white background of SSH is usable for me meaning that I can use it for hours without any problems.
Not sure if anyone has the same problem. I recently came across this online article on dark background vs white background. The phenomenon described in the article is similar to what I am experiencing except that I got headaches in seconds when reading on a dark background. It says "You should avoid using white text on a dark background when displaying paragraph text to make it easier for them to read. Forcing users to fixate on the white text for a long time can strain the user’s eyes. This is because white stimulates all three types of color sensitive visual receptors in the human eye in nearly equal amounts. This makes reading white paragraph text on dark backgrounds stressful on the eyes."
I am now using e-ink monitors (Dasung) and an e-ink cell phone (Hisense A2 pro). My world is black and white, but my quality of life is nearly normal. I cannot imagine how I had gone thru those years with led monitors. I still get problems when looking at things moving in e-ink monitors or reading on a dark background in e-ink monitors, but not at the same level on a led monitor and it is controllable.