How to position the checkmark in the center of the checkbox?

I'm struggling with the position of my checkmark from my checkboxes.

This is the code for the checkbox:

/* Style the tick */
.container .checkmark:after {
  left: 9px;
  top: 5px;
  width: 5px;
  height: 10px;
  border: solid white;
  border-width: 0 3px 3px 0;
  -webkit-transform: rotate(45deg);
  -ms-transform: rotate(45deg);
  transform: rotate(45deg);
}

The tick is always in the left corner and not in the middle. The position doesn't change whatever I write in "position:..."

I'm happy for any help.