Now press the "Edit shortcut - window" on the button with the icon.
In "Icon type - window" you have many options. We take the "Select from icon pack". Now you can see all installed icon packs. In our example it is "Buddy Theme 4 Apex Launcher". In the new window you see now all icons that were installed with the theme. Just pick one.
Now we are back in the "Edit shortcut - window". Here just press on "Okay". Done That's it