Icon customization in the panel

I like to change my icon for example for firefox in the top panel. What I have tried so far:

I have replaced the orig .png in .local/share/qubes-appmenus/VM/apps.icons and I changes the path in the .desktop file in dom0 and in the templateVM.
In the menu and and my desktop I can see the right icon I have selected, but as soon I start the app in the panel (xfce and kde) is shown the for example standart blue firefox icon for a blueVM.
I like to have in “my main using vm” that there are shown the icons without the vm color, like it is shown if u I open an application in dom0. I think I replaced all old.png files but anywhere at the filesystem there have to be the standard icon for the apps, that are shown in the panel.
Can u tell me this path and help me how to replace and customize it correctly?

Thank you very much:)
Ps: I know that there were some problems in Qubes 4.0x affecting to this , but that should be already fixed and I don*t know if it exactly these problems.