Hi Hilarion, I understand that you want to add a class to the body tag, and you want to add this class before the page finish loading (if I’m wrong let me know).
I don’t know why you need this for a page loader, but here it is (you can try this code and see if works for you):
<div class="pageloader is-active"></div>
<div class="infraloader"></div>
<script>
document.body.classList.add("class_two");
</script>
{{content}}