Mobile first, Sass, BEM, little JS for the form validation on click

Let me know how I did, first time doing 'mobile first'

Also interested if there was a better way of adding previous border colors apart from setting them individualy all over again (I wanted to remove red border for invalid input and revert to old border colors when the placholder is shown, feel like there may be some revert-like property I'm not familiar with). For reference, I haven't actually started learning JS yet.