Where can I find the system icons for Mail?
Obviously in macOS Sierra, Apple has changed how it packages some of the application resources and in this particular case the image assets that comprise the toolbar are still in Mail.app/Contents/Resources/, at least some of them, in the assets.car file as shown in the image farther below. Note that there are several .car files in the Resources folder, however looking at the open files for Mail.app in Activity Monitor, there are .car archives outside of the application bundle that are open as well, e.g.:

/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/VibrantLightAppearance.car
/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/SystemAppearance.car
/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/Assets.car
/System/Library/PrivateFrameworks/AccountsUI.framework/Versions/A/Resources/Assets.car
/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/FunctionRowAppearance.car



Note: Check in Activity Monitor on your system for a complete list of open files for Mail.app.


Doing a few Google searches, I found a few different apps, both GUI and CL, and had mixed results in both viewing and extracting assets from the .car archives. However, that was using only precompiled programs, and there are a few Xcode Projects I didn't test.

The one in the image below is ThemeEngine and while is actually showed more objects then Asset Catalog Tinkerer did, and opened some .car files that Asset Catalog Tinkerer couldn't, the drawback is ThemeEngine doesn't have an export function like Asset Catalog Tinkerer did. However, I could still copy and paste out of ThemeEngine though. Maybe the Xcode Projects would work better if I compiled then on my system, but just don't have the time to try.