I would try something like this but I don’t know if this change has any side effect. CSS is always the safest and easiest for change something. That is why I posted that.
I don’t remember what was the body class before the changes but I just replace it to archetype-regular instead of remove archetype-private_message class because there are some global css for [class*="archetype-"] class so we keep that in private messages too.
Header
<script type="text/discourse-plugin" version="0.8">
const body = document.querySelector("body");
api.onPageChange(() => {
if (body.classList.contains("archetype-private_message")) {
body.classList.replace("archetype-private_message", "archetype-regular");
}
});
</script>