@Michio Nakano You will get there exactly the icon as it is saved in Windows system files, i.e. shell32.dll and imageres.dll and they are very hard to change with home methods. It is certainly possible to find some method, but is it still worth the effort?
The icons should be here. C:\Windows\SystemApps\Microsoft.Windows.Search_cw5n1h2txyewy\Assets\Icons That said, what about removing the useless search crap altogether, and using something that actually works like Classic shell + Agent ransack?