index.css

input[type='number']{
    width: 70px;
    margin-right: 10px;
} 

input[type='number'][size="200"]{
    width: 25%;
    margin-right: 20px;
    margin-left:  10px;
} 

body {
    margin: 25px;
    background-color: rgb(248, 248, 248);
    font-family: Helvetica, sans-serif;
    font-size: 14px;
  }

.container[name = "flex"]{
    display: flex;
}

.drop-targets{
    position: relative;
    display: flex;
    flex-direction: row;
    justify-content: space-around;
    align-items: center;
    /*border: solid 1px rgb(0, 0, 0);*/
    margin: 20px 0;
}

.row{
    position: relative;
    /*border: solid 1px rgb(0, 0, 0);*/

}

/*background line*/
.row:before{
  content: '';
  position: absolute;
  top: 50%;
  left: 0;
  border-top: 2px solid rgb(0, 0, 0);
  width: 100%;
  transform: translateY(-50%);
}

.box {
    height: 20px;
    width: 20px;
    border: solid 3px rgb(204, 204, 204);
    margin: 10px;
    background-color: #000000;

    /* align items in the box */
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
}
canvas{
    position:relative;
    width: 100%;
    margin-bottom: -100000;
    left:0;
    top:0;
    z-index:-1;
    border: solid 3px rgb(204, 204, 204);
}
.hide {
    display: none;
}