I know I can increase the zoom or write a chrome extension to change it, but I think increasing HN's color contrast overall could help lots of readers.
That's one of many passive aggressive bits of operant conditioning pg originally added for quality control - meant to punish text-based posts by making them more difficult to read the longer they are. This is to discourage people from using text-based posts as an ad-hoc blogging platform, and make them as short as possible.
So no, that probably isn't ever going to happen.
This is a silly suggestion but perhaps the contrast could scale with the length? This would incentivize brevity and help (at least wrt shorter posts) those of us who find text posts difficult to see.
Remove or modify the redundant line about flag/hide/fav... etc. Put it on a right click, put it as a mouse-over or as a button. Don't use half the screen.
It is better design to eliminate redundant non-content.