Change the icon of all the folders on mac
Most standard system wide icons for the macOS are stored here:
/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources

The folder icon in which you're interested is named GenericFolderIcon.icns.

If you're running Mac OS X El Capitan (10.11) or later, due to System Integrity Protection, you won't be able to modify these files.

However, if you turn off System Integrity Protection, you'll gain the ability to edit the icon files. Here're instructions:


Boot into recovery mode (boot while holding down ⌘R)
Open the terminal
Type csrutil disable.
Reboot normally.
Now edit the icon file(s).
Repeat steps 1–4 to re-enable System Integrity protection, except that the Terminal command you'll use is csrutil enable.


Needless to say, be very careful when turning off System Integrity Protection and editing system files. There's a lot of potential to royally screw things up when doing so.

For step 5, you'll need some sort of program that can edit or generate ICNS files. Your best bet is probably to search around this site or with Google to find recommendations. You may have to try several options before you find something that fits your needs.