I use preservative-free (aka "artificial tears") eyedrops every few hours. They're generally used for lubrication/dry eyes and recommended if you're applying more than 4 times a day. Though not directly related to eye strain, dry eyes can contribute to it.
Additionally, I use a Chrome extension that notifies me every 20min that it's time to take a break. I like that it has minimal features (time adjustment, enable/disable) and is only active when it detects you're actively using the computer. https://chrome.google.com/webstore/detail/eye-blinker/fbgche...
I personally just use Stylus and have a custom css for making things a bit more readable and user friendly.
There's also this extension that just refines a couple things: https://github.com/plibither8/refined-hacker-news