Here's the css for him stolen directly from /japan/, I don't know what exactly you're using for dark mode so if you can inspect element and tweak it so he works I'll make the change.
body {
background:url(/asset/b/922b4c9c7a15040692d16a2eba7f82c468eb9f18e6dec6b3a7dbc03ef6664e65.png) bottom right fixed no-repeat, linear-gradient(var(--background-top) 3em,var(--background-rest) 230px);
background-size: 25vmin;
}
Otherwise you can try blocking him with an adblocker.