1) Open Nautilus
In Nautilus file browser, right click a folder icon, select Properties.
2) Edit Icon Settings
Under the Basic tab, click the icon image to open up the “Select Custom Icon” window.
3) Search for Icon Image
In the Location field, type
/usr/share/icons, press Enter. (You can hide or unhide the Location field by clicking the Edit button on the top-left of the window.)
Watch this subfolder as well:
4) Choose Icon Image
Browse and select an icon you want.
5) Save Changes
Click the Open button on the bottom-right of the window to confirm.
6) Close Properties window
Click the close button to close the properties window and finalize the new settings.
Also have a look at applications icons
You can alternatively download and set what you like, for example from here:
Here are some example icons can be downloaded: