That's a cool idea, but it's not a substitute for meeting basic accessibility and readability needs. HN violates the WCAG accessibility standards for text contrast (your browser dev tools will happily point out which text is affected), it ignores platform fonts in favor of an outdated font stack that doesn't kern properly on many Linux devices, and it displays line lengths of up to 200 characters, even though research suggests that CPL should not exceed 70.

These are absolutely basic things, and fixing them should not require individual users to fiddle with CSS – even on a website called "Hacker" News.