A way to enable custom 404 for Tumblr. Well, it 'kinda' works

custom_404_for_tumblr.js.htm

<script type="text/javascript">
/* Works for me */
	var text_posts = document.getElementsByClassName("regular");
	var text_404 = "The URL you requested could not be found.";
	var title_404 = "Not Found";
	if(text_posts.length == 1){
		var bodyNode = text_posts[0].lastChild;
		if(bodyNode.previousSibling.textContent == text_404) {
			// titleNode.innerHTML = "<a href='/'>Not Found</a>";
			var blog_loc = "http://" + document.domain + "/";
			var query = window.location.href.slice(blog_loc.length);
			var tokens = query.toLowerCase().split('/');
			var keyword = tokens.join(" ");
			var bodyContent = "Looks like you came from an old bookmark, or just looking for a page that doesn't exist. Were you looking for <span class='tg'><a href='/search/"+ escape(keyword) +"'>"+keyword+"</a></span>";
			bodyNode.previousSibling.innerHTML = bodyContent;
		}
	}
</script>