.parent_container {
  --gap: 15px;
  --num-cols: 30;
  --row-height: 10px;
  
  box-sizing: border-box;
  margin:0px;
  padding: var(--gap);
  display: inline-block;
  
  display: grid;
  grid-template-columns: repeat(var(--num-cols), 1fr);
  grid-auto-rows: var(--row-height);
  gap: var(--gap);
  
  border: solid gray;
}