Not much is read from the screenshots shared on Twitter. However, it seems that in the future users will be able to customize some of the colors in their chat boxes. For now, however, it seems that only different, light and dark green tones can be chosen here.
According to WABetaInfo, the feature is currently only under development, so nothing can be said about a possible release period. It is also unclear whether it will be available for iOS, Android, or even both operating systems.