Ah, you have a button for the toggle! Did you know that you can conform to the user's light/dark preference with CSS? Like this:

    :root { --varname: #FFE; }
    
    @media (prefers-color-scheme: dark) {
        :root { --varname: #022; }
    }

Edit: To expand on this, I dislike light/dark toggles. If I have dark mode on, seeing a bright screen and having to search for a toggle is jarring. Just show me the colours according to my preference!

Last I checked on this the workaround is embedding CSS inside an SVG, here's a demo: https://github.com/sindresorhus/css-in-readme-like-wat

I used this on a personal project to add a README logo that is dark mode friendly: https://paul.af/github-readme-dark-mode