How to make react app with pixel sizing adapt to different screen sizes?

Question

I've been making a website with react and I've been using css with classNames. I also was following a figma document where the figma screens were a certain size lets say x pixels by y pixels. But, on other computers, it might have a larger or smaller pixel dimension (a pixels by b pixels). I've used the x by y pixel for div dimensions as well as button sizes and more. So, when I open my application on other computers with a larger pixel dimension, a portion of the screen is just white, and it still displays the website as if it were on a smaller screen. Is there a way to fix this so that even though I used pixel values my app will adjust to different sized screens? Or, do I have to go back and change everything to percentages?