body
{
  font-family: Arial, sans-serif;
}

#mainDiv
{
  display: flex;
  justify-content: center;
  align-items: center;
  height: 500px;
  width: 600px;
  background-color: lightgreen;
  border: 1px solid blue;
  border-radius: 10px;
  margin: 0 auto;
}

.item
{
  border: 2px solid green;
  border-radius: 5px;
  /*width: 400px;
  height: 200px;*/
  background-color: rgb(109, 224, 244);
  padding: 2em;
}