Intro component with sign-up form[HTML, SCSS, Vanilla JS, MobileFirst]

I learned to manage the mixins for mobile settings, but when I moved to desktop view I found that I couldn't move the elements around like I wanted. For example, setting something to a certain position only worked for one desktop width. So to solve this, should I have written mixins for every possible desktop width that the site in question would be displayed on? or is there a more general solution?