Ian

I'm adding a screenshot of my Niagara button and another screenshot of the e-mail app that is attached to the button.  

To make it clear: eM Client is my e-mail app and definitely not an ebook reader.

This doesn't work for the Niagara button. There I can only change the underlaying app but not the icon.

This is how the Niagara button looks like when I'm using Gmail as underlying app. With "eM Client" it should look the same. 

No. It's not about the theme, it's about the app category. Niagara choose the wrong category for my e-mail app.