Manage Site Theme - 7.6 upgrade to 10.3.2.7882

Upgrading from 7.6.22.49539 to 10.3.2.7882. We are using a 7.6, fij-based theme. "Allow user-customization of the site home page" is enabled.

Manage > Manage User Theme > Manage Site Theme results in a 404. The URL generated looks like this: 

mysite.com/.../editpage

In an out of the box, 10.3 instance with a similar configuration, this url navigates to a page that says:

Page Editing

You are managing a theme but have not selected a page to edit. This content is provided as a placeholder for editing headers and footers until a specific page is requested to be edited.

How can we get our upgraded instance to display the "placeholder" page like the 10.3 vanilla instance instead of displaying the IIS 404 page?