No, I don't know "those titles that change randomly on every page/refresh". Could you provide a link to an example? Some wikis change the display title but that is not the same as changing randomly. What would even be the purpose of doing that?

This should work quite well.

JS (MediaWiki:Common.js):

var images =
[
    "Link to img",
    "Link to img",
    "Etc."
]

var image = images[Math.floor(Math.random() * images.length)];
var stylesheet = mw.util.addCSS("body { --theme-body-background-image-random: url(" + image + "); }");

CSS (MediaWiki:Common.css):

.page__main {
background-image: var(--theme-body-background-image-random);
background-repeat: no-repeat;
background-size: cover;
background-position: top;
background-attachment: fixed;
}

^^https://marvel.fandom.com/wiki/Marvel_Database