A user over at XDA put together a little modification that gives the Galaxy S8 and S8+ a taste of the Pixel's navigation icons. It's just an overlay, and it does have a couple of small bugs of its own. But, it is a basic and straightforward little feature that can improve your aesthetic quality of life. At least, if you find Samsung's stock look is a bit too busy for your tastes.
There is one small bug. On tap, the home button is briefly changed back to the original icon, so things aren't quite seamless. The back and recents buttons don't have the same problem, and even when during interaction they keep their modified appearance. It isn't a huge change, but if the stock icons have been bothering you, the option is there.
You can download the APK for the overlay here, or at XDA via the source link below.