Add a page outside of the current theme?

Is it possible to add a page that is totally blank and doesn’t use any components of the site’s theme? Or is the best way to do that to upload it separately to the web server outside of WordPress?

Edit: Updated with the added context from your comment.

You could do this with a custom Page template. In your theme, create a new file. In this example, my theme will be “mytheme2015,” my new file is “template-holiday-card.php”:

in wp-content/themes/mytheme2015/template-holiday-card.php

<?php
/**
* Template Name: Holiday Card
*/

Now, once you have that template file created with a header like that, you can use it in the WordPress admin when you create a new Page type. The commented out “Template Name:” part tells WordPress what to call the template in the backend.

Note: if you don’t see the Page Attributes box, look in the Screen Options pulldown menu at the top of the page.

Now, you can put whatever code you want in template-holiday-card.php which (since it’s blank) won’t display anything by default.

If you want to allow your designer to input the HTML/CSS directly in WordPress through the admin, you could just have a call to the_content() in your template and allow them to edit the page’s content in HTML mode. Your resulting template would look something like:

<?php
/**
* Template Name: Holiday Card
*/

the_content();