Apple provide a system-wide automation system which can be used with Add Folder Icons to automate addition of icons to new folders. This Help page provides a walk-through guide to setting this up. For more detailed technical information on Add Folder Icons automation, see the AppleScript help section.
The Finder can run AppleScript scripts when changes happen to certain folders. These are referred to as "Folder Actions". You can use these to instruct Add Folder Icons to automatically apply an icon style to a new sub-folder when it is added to some containing folder. The exact way to set them up varies with Mac OS X version, so you might have to dig around in Finder menus a little bit if you cannot find the exact entries described below. These instructions here are written for macOS X 10.13, "High Sierra".
At the time of writing, Apple host a page here with general details about setting up Folder Actions and a related page here with technical details. These might be useful if you get stuck.
Here we add icons using the "Classic" preset to any non-application folders added to some containing folder. The first part just makes sure that things the script is considering really do seem to be simple folders, while the second part does the actual work of taking the list of added items from the Finder and sending them on to Add Folder Icons. Right at the end, you can see the preset name chosen - you can replace this with different presets of your choosing.
-- This is a hacky but workable way of determining that a Mac alias to a -- file or folder is indeed a folder (ends in special character ":") but -- not an application (ends in ".app:"). -- on isNonAppFolder(theItem) set isFolder to false try set theAppExtension to characters -1 thru -5 of theItem as string on error set theAppExtension to "" end try if theAppExtension is not ".app:" then set theLastCharacter to character -1 of theItem as string if theLastCharacter is ":" then set isFolder to true end if end if return isFolder end isNonAppFolder -- This is the main folder action, which tolerates additions of non-folder -- items such as files or applications and ignores those. -- on adding folder items to theAttachedFolder after receiving theNewItems set thePOSIXFolderPaths to {} repeat with theItem in theNewItems if isNonAppFolder(theItem as string) then copy {POSIX file (POSIX path of theItem)} to end of thePOSIXFolderPaths end if end repeat if length of thePOSIXFolderPaths is not 0 then tell application "Finder" activate end tell tell application "Add Folder Icons" apply "Preset: Classic thumbnails" to thePOSIXFolderPaths end tell end if end adding folder items to
To use this script:
Successful script creation
You might want to change the line near the end to apply a different preset - e.g. "Preset: Square covers (e.g. CDs)" if you're setting up a folder to hold folders of music albums. Any name copied exactly from the Add Folder Icons "Manage Styles" window will do.
Saving the script into the correct location has been made quite difficult on modern versions of Mac OS X. Press Command+S (or navigate to File -> Save…). In the "Save As" filename field, type the "~" character (on may Mac keyboards this is at the top left, just underneath the physical, or touch-bar-based Escape key). If you get it right, rather than "~" appearing, a "Go to the folder:" prompt appears. Continue typing to enter, "~/Library/Scripts" (note carefully the forwards slashes, including the one just after "~"):
"Go to the folder"
Press Return. We're almost done, but you need to save your script inside one more nested containng folder for it to live in the correct "magic location", but this folder isn't always created in new, clean user accounts. To make sure, click on the up/arrow just to the right of "Save As" field to expand the dialogue box to full size. The window should look at bit like this:
Expanded "Save As" view
If you can see a "Folder Action Scripts" folder already there, then just double-click on it. Otherwise, click on the "New folder" button as shown above and in the pop-up window that appears, enter the precise text, "Folder Action Scripts" (capitalised as shown):
New folder
Whether you had to create that last folder or whether it already existed, you should now have your "Save As" window ready to go. Give the action a meaningful name - e.g. "Add - Classic Icons", if that's appropriate for the preset your Folder Actions script applies - and you're almost ready to save. First make sure you really are inside that "Folder Action Scripts" location by double-checking the location name shown to the left of the search box, and that the "File Format" selection towards the bottom left is "Script", both as shown in the screenshot below:
Final name
In the Finder, navigate to a folder which is going to contain other folders that need to gain icons. For example, if you had folders full of music albums and wanted to automatically change their icon to album art whenever you added a new album folder, then whatever containing folder - "Music", "Albums" or similar - would be the one to choose. Ctrl+Click (secondary/right-click, two-finger tap if configured) on this folder. A menu for that folder opens. At or near the bottom will be "Services". In the submenu from there, select "Folder Actions Setup...". A small window opens, which will already have added the folder you clicked on:
Folder Actions Setup selection
Choose the action you saved earlier. Make sure the "Enable Folder Actions" check box at the top left of the window is enabled:
Folder Actions Setup complete
That is, at last, all. You can close the setup window. When you add folders inside the container you selected, Add Folder Icons will be invoked automatically for them.