Responsive Web Page using HTML and CSS

Hello, I would like to know how to style input field error message in this webpage. I couldnt do that hence I have given a label here. I tried few things like using input:required:invalid and then making visibility:visible; but couldnt achive the desired output. Another problem that my solution has is regarding the background svg for width > 1440px as I gave position for the background image: top left; I would love to know how can I make this webpage better. Inputs are appreciated. Thanks in advance!