Your eyes evolved for seeing. They're incredibly intelligently designed for that purpose. No choice of colour, no lighting, no shapes or content will have a deleterious affect on your eyesight. You mom was wrong.
Focusing at desktop screen distance for hours at a time has never been shown to affect long-term ability to focus, despite the truthiness of what you may have read in opinion pieces on the internet. It doesn't hurt to take a break for all kinds of reasons, but avoiding eye damage is not really one of them.
Some people get eyestrain when trying to focus under certain circumstances. Eyestrain is in fact the muscles around your eyes, not your eyes themselves, and often is a result of squinting in order to (a) adjust the shape of the eye or (2) reduce the aperture into effectively alter internal focal length. Both of those should be alleviated by corrective lenses. What that generally means is you have a preexisting condition that you're trying to compensate for, not a condition caused by staring at your screen. Get your eyes tested.
It's a fact that most people's eyesight deteriorates over the years. I keep getting my bifocals strengthened, and I find I now need them to use a lot of computer screens for work (damn high-res displays, get off my lawn). Your eyesight is not the only thing that deteriorates when you hit your 50s though, and it's all been happening since long before computers screens were invented. There is no correlation between computer use and deteriorating eyesight.
TL;DR don't worry about it, you can do it as much as your want and it's not going to make you go blind.