As an ex-Marine sniper I use a black background with green bars. Before taking the shot we were trained to focus on something green to rest the eyes. And many years of screen time has proven that to be a good combination.
....whereas I have to study a chart that is colored. .
An optometrist I know suggests to exercise the eyes to help prevent eye strain.
every 20 mins or so refocus on different distances - even if done quickly, its a good habit, and keeps them healthy.
That's good advice. When you guys get older you'll find that it takes longer for your eyes to refocus.