1) Download and install the Xposed framework.
2) Download and install Unicon from the Play Store. ($2.00 USD)
3) Reboot after selecting the Xposed module and enabling it.
4) Open Unicon from the app drawer, select an icon pack, and enjoy!
The way this works is that Xposed allows Unicon to change the application icons across the entire system, so it'll even show up in the small app icons in places (such as the settings cog inside of the Settings app, at the top). The only con to this method is that it isn't possible to enable ART along with this tweak, since it relies on Xposed. If anyone knows of a way to do this without Xposed, please let me know! I hoped this helped!
(Attached is a screenshot for proof that it does indeed work on the GEL/GNW, icon pack used is Click UI, $2.00 USD)