Fixed allign

No margins and transform

HTML

<div class="full container fixed">
  <div class="modal">
    <h1>
     Here!
    </h1>
    <div>
      Is contenent here!
    </div>
  </div>
</div>

CSS

.full { top:0;left:0;right:0;bottom:0; }
.fixed { position:fixed; }
.container { display:flex;  justify-content: center; align-items:center; border:1px solid red; }
.modal {border:1px solid green;}