The blindingly-bright folder icons in Yosemite, El Capitan, and Sierra can all be replaced with darker versions using the method I describe here:
http://www.columbia.edu/~em36/yosemitefoldericons.html
As described in the text file included with the icons, you need to boot into Recovery mode, temporarily disable a security feature, reboot back to macOS and replace the icons using the free LiteIcon utility, then boot again into recovery mode to re-enable the security feature. (If you're feeling paranoid, turn off wi-fi when booting with the security feature disabled.)
This method works perfectly in Sierra, as it does in previous versions.
For further details, see this thread elsewhere on the forum:
https://forums.macrumors.com/threads/darker-versions-of-yosemite-folder-icons.1816561/
http://www.columbia.edu/~em36/yosemitefoldericons.html
As described in the text file included with the icons, you need to boot into Recovery mode, temporarily disable a security feature, reboot back to macOS and replace the icons using the free LiteIcon utility, then boot again into recovery mode to re-enable the security feature. (If you're feeling paranoid, turn off wi-fi when booting with the security feature disabled.)
This method works perfectly in Sierra, as it does in previous versions.
For further details, see this thread elsewhere on the forum:
https://forums.macrumors.com/threads/darker-versions-of-yosemite-folder-icons.1816561/