Tip-Calculator With CSS Grid, CSS Flexbox, Vanilla Javascript

I think I did really good on this project. I had a very hard time trying to put dots inside the zeros and am not sure how to do it. Also, never figured out how to add colors to the dollar images on the right side of the app. This was my first time using Javascript but I feel like maybe I could have had a better solution with that. But out of all of this, the most important I want to know is how to make this compatible with mobile devices(which I know requires media screen with CSS). Any suggestions??