this is undocumented, and certainly the developers may change the implementation but try this that I worked out trial and error:
using 1.7.12 on El Capitan
Locate this file:
Select Show package contents on this TotalFinder.bundle
In this location:
there are two files: icons.plist and icons2.plist
icons.plist is used for Yosemite
Icons2.plist seems to be specific to El Capitan.
AFTER MAKING ANY CHANGES TO ICONS2.PLIST, RUN KILLALL FINDER,
forces TF to reload config and Finder to reload, otherwise change in icons won’t take effect. Changes I’ve made below, in any of the examples, where I redirect to a different icns file in the CoreType.bundle/contents/resources folder, those are my CUSTOM icns file that I placed there over writing the default OS X. You do NOT have to use the default icns files in the Coretypes.bundle/contents/resources folder, you can replace them or point to a totally different icns in a different location, TF will load using the path you insert, just make sure the icns files are properly formatted and comparable in resolution to the original OS X ones (1024x1024, 72 dpi, and have all the proper other resolutions in the icns (512x512, 256x256, etc)
For the most part the String of each entry defines the location of the folder and the Key defines the path to the icns file to display if that folder is visible in the sidebar. For me, that’s backward, I would have swtiched Key and String, but that’s just me. Good luck! Sorry BinarySteve if I spilled the beans, but actually Darwin pointed me in the right direction in a post he made back in Oct 2015.
PS , the examples below, there are supposed to be key and string XML embedded tags as part of the code sample, but when I posted these got stripped out because I think the editor is interpreting them as HTML formatting tage in the post, just remember the first line is the key value and the second line is the string value
For Desktop icon, Lines:
This now shows the icon DesktopFolderIcon.icns for the Desktop in the Sidebar.
For Music Folder icon Lines:
For Application Folder icon Lines:
For Pictures Folder icon Lines:
For Movies Folder icon lines: