gist-to-printable.css

*{
    background: white!important;
    color: black;
}
.markdown-body code {
    white-space: pre-wrap!important;
}
.markdown-body pre {
    border: solid 2px;
}
.markdown-body h1,
.markdown-body h2,
.markdown-body h3,
.markdown-body h4,
.markdown-body h5,
.markdown-body h6 {
    page-break-after: avoid;
}
.markdown-body img {
    width: 80%;
    display: block;
    border: solid 2px;
    border-radius: 6px;
    margin: 1rem auto;
}

gist-to-printable.js

(async () => {
    const fn = document.querySelector(".form-control.js-gist-filename.js-blob-filename")?.value;
    if (!fn?.endsWith(".md")) {
        console.log("Not a markdown");
        return;
    }
    const edit = document.querySelector("button.code");
    const preview = document.querySelector("button.preview");
    if (edit === null || preview === null) {
        console.log("unexpected page structure: tab switching button(s) not found.");
        alert("Internal error occurred.");
        return;
    }
    preview.click();
    await new Promise((res) => setTimeout(res, 0));
    const markdown = document.querySelector(".commit-preview.markdown-body");
    if (markdown === null) {
        console.log("unexpected page structure: preview body not found.");
        alert("Internal error occurred.");
        return;
    }
    const wnd = window.open(".");
    if (wnd === null) {
        console.log("failed to open new window.");
        alert("Internal error occurred.");
        return;
    }
    window.focus();
    await new Promise((res) => setTimeout(res, 2500));
    wnd.document.body.innerHTML = markdown.outerHTML;
    wnd.document.body.firstElementChild.style.width = "210mm";
    let sty = wnd.document.createElement("style");
    sty.innerHTML = "*{background:white!important;color:#000}.markdown-body code{white-space:pre-wrap!important}.markdown-body pre{border:solid 2px;white-space: pre-wrap!important}.markdown-body h1,.markdown-body h2,.markdown-body h3,.markdown-body h4,.markdown-body h5,.markdown-body h6{page-break-after:avoid}.markdown-body img{width:80%;display:block;border:solid 2px;border-radius:6px;margin:1rem auto}";
    wnd.document.head.append(sty);
    wnd.document.title = fn.replace(/\.md$/, ".pdf");
    try {
        wnd.document.querySelector('link[rel="stylesheet"][href*=dark').href = wnd.document.querySelector('link[rel="stylesheet"][data-color-theme*=light]').getAttribute('data-href')
    } catch {}
    console.log("PDF is ready.");
    wnd.focus();
    edit.click();
})();