im having trouble centering a scrollbox

i want the scrollbox to be in the center no mater what the monitors size is how would i go about that?

im really new to html

here is the code i used for said scrollbox:

    <style>
      .scroll {
        background-color: black;
        width: 700px;
        height: 700px;
        overflow-x: hidden;
        overflow-y: auto;
        text-align: center;
        padding: 20px;
        box-shadow: 0rem -0rem 0.5rem 0.5rem #20ff00;
        z-index: 4;
        position: relative;
        margin: auto;
        margin-top: relative;
      }
      </style>