Responsive Tip Calculator using grid, flex and some javascript

Hey guys, I hope you like it. I am really in desperate need for all your opinions and suggestions if you have the time to write them. I am basically a C++ developer but I was fascinated by the web for so long that finally I decided to learn it. I am bad at interpreted languages so my JavaScript code may seem rough and messy but I really poured everything I knew about JS in there. Same thing for CSS, it's a new rabbit hole for me but I'm getting used to it and again I would really be grateful to anyone who leaves some suggestions and/or remarks. Thank you again everyone, and Happy Coding :D !!!!