Tip-Calculator-App(HTML/CSS/JS)

This is my solution for the challenge still missing 2 things:

1. Button/Div of 5%,10% etc when clicked need to change color indicating that it's showing 5%(see designs), i know the css class can be used to change with JS but i cannot understand how to revert it(if another percent like 15% is selected it needs to change color but the previous 5% should change back to it's previous color), i can't figure out how to do it.
2. Reset button should be disabled when there is no input given, i don't know how to check changes for each input and switch between enabled and disabled.