Dark Reader, often recommended, is very slow, especially on old hardware. This built-in mode is much more efficient and works well on vast majority of pages.

Use it with Stylus

https://news.ycombinator.com/item?id=23199062

See also:

dang + "dark mode": https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

dang + "on the list" (lol): https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

I know this sounds like an ad, but I was pretty impressed with it

    # HN dark Mode:
    news.ycombinator.com##body:style(background: black)
    news.ycombinator.com##td:style(color: #fafafa !important)
    news.ycombinator.com##table:style(background-color: #24273a)
    news.ycombinator.com##div.toptext:style(color: #fafafa)
    news.ycombinator.com##div.c00:style(color: #fafafa !important)
    news.ycombinator.com##a:style(color: #ffa000 !important)
    news.ycombinator.com##span#karma:style(color: #faa000 !important)
    news.ycombinator.com##span.pagetop:style(color: #fafafa !important)
    news.ycombinator.com##textarea:style(color: #696969 !important; background: inherit; )
    news.ycombinator.com###hnmain:style(background: #24273a !important)

    body {
        background-color: #262626 !important;
    }

    body > center > table, input, textarea {
        background-color: #222 !important;
    }

    body > center > table > tbody > tr:first-child > td {
        background-color: #ff6600 !important;
    }

    /* Bright text */
    td.title a:link, span.comment font, span.comment font a:link, u a:link, span.yclinks a:link, body:not([id]),
    td:nth-child(2):not(.subtext) > a:link, input, textarea, p > a, a > u, .c00, .c00 a:link,
    a[href="http://www.ycombinator.com/apply/"], a[href="https://www.ycombinator.com/apply/"] {
        color: #ccc !important;
    }

    .admin td {
        color: #aaa !important;
    }

    /* search box and comment box */
    input, textarea {
        border: 1px solid #828282 !important;
    }

https://chromewebstore.google.com/detail/hn-dark-mode/abpjfl...

It has a "Dark Mode" under options, plus it allows you to revisit a thread later and know which comments are new.