If you want to inject CSS, use a "user styles" manager like Stylus. There's almost no learning curve if you know CSS.

You can write entire sheets for all pages, multiple or single domains. For example to make HN legible (basic text, spacing and contrast):

    @-moz-document domain("ycombinator.com") {
        :root {
            --font-size: 1.2rem;
            --text-color: #333;
        }
        .comment {
            font-size: var(--font-size);
            line-height: 1.5;
        }
        body {
            font-family: sans-serif !important;
            font-size: 100%;
        }
        .comhead a:link,
        .subtext a:visited,
        .c00,
        .c00 a:link,
        td {
            color: #333;
            font-size: var(--font-size);
        }
        td {
            background-color: transparent;
        }

}

(Personally I remove all fading from downvoted comments as well, they're unreadable)

https://chrome.google.com/webstore/detail/stylus/clngdbkpkpe... https://userstyles.org/