Google Messages (the new default messenger for samsung devices) allows you to select conversation colors and matches your device's color pallete. Unfortunately, fonts cannot be changed.
I would not anticipate any new features making their way to Samsung messages as it is slowly being phased out.
And, that's just the way it should stay. I don't need to be horrified by other people's font and color choices. I remember that annoyance from the AOL Instant Messenger days.
