CSS Chat App - with working messages! 💬🎉

I've extended this one with a working chat function, which I was pretty pleased with. Any suggestions on how I could have made this simpler v much appreciated though! 😃

Nothing else with this one that's still bugging me (although maybe there's an issue I just haven't spotted! 😬), but any general adjustments, simplifications or suggestions for improvements would also be really helpful.