Android: If you have a rooted device with software keys, you can customise your navigation bar’s icons with SoftKeyZ. The app includes a batch of stock icons or you can bring your own.
Some custom ROMs have this ability built-in, but if you want to swap out your icons without flashing an entirely new ROM (or if you have and yours doesn’t support it), this app will do the job just fine. Along the top you’ll see the master icon pack, and below is a scrollable list of options. The app costs $2 and is available on the Play Store.
SoftKeyZ ($2) [Google Play]