How to remove the default footer and add a new one?

What I’m trying to do

Remove the default footer and replace it with a new one.

Things I have tried

/ == Site footer ==

document.getElementsByClassName('site-footer')[0].innerHTML = 'Your text or HTML goes between the single quotes.';

and

.site-footer {
    /* hides "Powered by Obsidian Publish" */
    font-size: 0px;
}

.site-footer:after {
    /* add you text */
    font-size: 16px;
    content: "Your footer is here. **Bold**. <b>Bold2</b>." 
    /* Markdown and HTML don't work. You mgiht be able to add an image, but I didn't test this: https://developer.mozilla.org/en-US/docs/Web/CSS/content  */
}

The existing footer can be removed but the new footer doesn’t show up.