Whenever the folks at YC get around to adding the feature. To be honest, I don't think they put too many resources into HN, so that could be a while. If ever.

It's an often requested feature, but there's no timeline. The HN search (search box at the footer of all pages) has dozens of previous discussions. https://news.ycombinator.com/item?id=36342085 for example lists multiple workarounds. I use the darkreader browser extension for example.