Steps to Reproduce:
1. Using the Plugins SDK, create and deploy a new theme named foo-website like this:
2. Using the "Manage Page" feature, set the current theme to be "Foo Website"
If the bug still exists, then Chrome will report 404 errors like the following:
It turns out that "-web" is enough to cause the problem. So perhaps this is causing the portal to somehow get the theme confused with "Web Plugins" which have a "-web" extension. Just a guess though.
Obvious workaround: Don't put "-web" in your theme name