Go -> Go to Folder
/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/
Look for the GenericDocumentIcon.icns file. Put one copy on your desktop (for editing) and another somewhere else (for backup). You can then open the desktop copy in preview, save as a .png, edit / change / whatever, save again. Change the file extension. Put that new file in the same folder, overwrite, restart. Should work. Here’s where I got this info:
https://stackoverflow.com/questions/3454201/mac-os-x-document-icon-template
https://osxdaily.com/2014/07/27/mac-os-x-system-icons-location/
Do this with care. Both of these sites say this is a little advanced, FYI. Read those pages first!
Not my fault if something gets messed up! Good luck!
—
Tyler Chittick
Integration & Warehouse Manager
Technical Support
Maxx Digital
http://www.maxxdigital.com
tyler@maxxdigital.com
714-374-4944