How to customize tumblr 404 URL not found page

Question

It seems Tumblr does not provide a div tag to customize the look and feel of the 'url not found' page. Is there any way other way to customize the 404 (URL NOT FOUND) Page?

Answer 1

Edit: this method only works on tumblogs that don't have pages.

Ask, Submit, and other pages created in the Customize section will be detected as "not found" with this code.

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Description

Tumblr uses the normal {block:Posts} loop for static pages but without assigning any variables like {PostID}. If we use a class like post{PostID}, on 404 pages all static pages we'll see a .post element, whereas on posts the elements will be something like .post125678

Example

{block:Posts}
  <div class="post{PostID}">
    {block:Photo}all your blocks here{/block:Photo}
  </div>
{/block:Posts}

Using javascript:

var is404 = document.getElementsByClass('post').length;

Using CSS:

.post {
    /*this is a 404 page, customize it*/
}

Cool javascript-less example

{block:Posts}
  <div class="post{PostID}">
    {block:Photo}all your blocks here{/block:Photo}
  </div>
{/block:Posts}
<div class="fill-me"></div>

In CSS:

.post { /*Hide Tumblr's 404 message*/
    display: none;
}
.post + .fill-me:before { /*use your message and style*/
    content: 'This page was not found!';
    font-size: 2em;
}

Edit: Possible fixes

To fix this method, we should find a {tag} that is only shown on pages but not on 404 pages.

{ShortURL}, if it weren't buggy, could be used since in theory 404 pages shouldn't have a ShortURL.

I also tried {Permalink} but it behaves like {PostID} in this case.