How can I edit some app icons not set by gnome-tweaks?

I changed my icons through gnome-tweaks. Though some system, third-party applications are not changed. Before when using other linux distro, I manually change this in a desktop entry or something (can’t remember now). How can I do this manual setting of icons in NixOS? I’m not using home-manager, only configuration.nix. Can also suggest an alternative way of setting icons.