Works using math CSS injection [1]:

    ```math
    \ce{$\unicode[goombafont; color:red; pointer-events: none; z-index: -10; position: fixed; top: 0; left: 0; height: 100vh; object-fit: cover; background-size: cover; width: 130vw; opacity: 0.5; background: url('https://github.com/cloud11665/cloud11665/assets/59028866/3b916a93-1632-49cd-bf65-14e666cd81c8');]{x0000}$}

[1]: https://raw.githubusercontent.com/cloud11665/cloud11665/mast...

Yeah, it seems like MathJax just puts whatever is in the square brackets of the Unicode tag into css font-family without escaping it beforehand