When I want to use a site in darkmode, I always use this code as a bookmarklet:

    s=document.createElement('style');
    s.textContent=`
        * {
            color: #2c2 !important;
            background: #222 !important;
            font-family: arial !important;
        }
    `;
    document.head.appendChild(s);

I use a lot of bookmarklets, so I wrote a tool to convert clean code to bookmarklets and vice versa:

https://www.gibney.org/bookmarklet_editor