title: Flex box width different for desktop and mobile
body: <p>I'm having a form with some mat-form-fields. Now I've set the width of those items to 750px, but this is of course not correct for a mobile view.</p>
  
  <p>I'm wondering how I can put it the form or mat-form-field that if the page is view on desktop that it should be with a width of 750px, otherwise a width of 100%.</p>
  
  <p>I tried with flex-grow-gt-xs, but nothing.</p>