Let's follow up this discussion at the Neptune Community
(this forum will be kept in read-only mode)
Known issue: when creating an Extension App the CSS MIME object is not created.
Workaround: You will need to Activate once setting the Application Type to "Application". This will create the CSS MIME object for the Extension. Once the Extension App is active, please switch back to "Extension", activate again and go on working.
Switch to "Application" (Application Type radio button) and activate. This will create the missing MIME object (CSS) for you.
Once you have activated, you can switch back to "Extension", activate again, and go on working.
The CSS will look fine after that.