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 :)

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...