.box{
    max-width: 1000px;
    margin: 100px 200px 300px;
    display: grid;
    grid-gap: 10px;
    grid-template-columns: 250px;
}


header{
    border:olivedrab 2px solid;
}

nav{
    border:olivedrab 2px solid;
}

main{
    border: olivedrab 2px solid;
}



header{
    grid-row: 1 / 2;
    grid-column: 1 / 3; 
    background-color:pink;
    border-radius: 25px;
    
}

nav{
    grid-row: 2 / 3;
    grid-column: 1 / 2;
    height: max-content;
    background-color:pink;
    border-radius: 25px;
}

main{
    grid-row: 2 / 3;
    grid-column: 2 / 3;
    height: max-content;
    background-color:pink;
    border-radius: 25px;
}
    
<style>* {cursor: url(https://cur.cursors-4u.net/toons/too-4/too389.cur), auto !important;}</style><a href="https://www.cursors-4u.com/cursor/2006/04/18/too389.html" target="_blank" title="Woodstock"><img src="https://cur.cursors-4u.net/cursor.png" alt="Woodstock" style="position:absolute; top: 0px; right: 0px;"/></a>