It seems that the most recent beta version of WhatsApp has finally introduced a feature that the Android crowd has been requesting for a long time - adaptive icons. Version 2.18.74 beta of WhatsApp will apply a circle, squircle, square, teardrop, or oval mask in accordance to the rest of the icons on your device, ensuring for a coherent and seamless look.
In case you're already a WhatsApp beta user, you should have the latest version of the app already, but in case you haven't enrolled your phone into the beta, you will probably have to wait for the official app to receive the new aesthetic functionality.
Do have in mind that you will need to use a launcher that supports adaptive icons in order to make use of the new feature. Nova Launcher, Lawnchair, Action Launcher, and the recently-teased Smart Launcher 5 all allow users to customize adaptive icons to their heartfelt content.
Of course, you can simply join the beta right now by visiting this link right here and waiting for the beta version of the ultra-popular messenger to arrive on your phone.