APEX 5 Theme/Static files issue (503 error)

I’ve recently come across the issue of static images not working and my custom theme roller theme not being applied to my APEX 5 app.

When looking at the browser’s console, I noticed a bunch of errors:
Failed to load resource: the server responded with a status of 503 (Service Unavailable)

After digging around some and scratching my head for a while, the issue turned out to be expired APEX_REST_PUBLIC_USER and APEX_LISTENER database accounts.

When the passwords were reset for these accounts, the images came back and the custom theme was applied.