Change icon of application (Almost there...)

Hi everybody.
Maybe someone can help me :)

I'm trying to change the icon of an application to better suit the windows 10 look, the app is the Postbox email client.
So far I used Resource Hacker and I almost did it, I changed the original icon, the shortcuts in the taskbar but as soon as I open the app the "running app" icon reverts to the original one, any help on changing that icon to?

Thanks for any help.

I attached some screenshots, the last one shows the original icon as soon as I open the app. That's the one I want to change.

Just solved it, there are another folder in C:\Program Files (x86)\Postbox\chrome\icons\default where the app keeps 4 .ico files, just changed those to my version and everything looks good now :)