Unity version: 2020.1.0f1
Icons what can load using EditorGUIUtility.IconContent
You can change script icon by file id
- Open
*.cs.meta
in Text Editor - Modify line
icon: {instanceID: 0}
toicon: {fileID: <FILE ID>, guid: 0000000000000000d000000000000000, type: 0}
- Save and focus Unity Editor