.boxes-container {
  max-width: 800px;
  margin: 100px auto;
}
.boxes input{
  margin: 0;
  appearance: none;
  -webkit-appearance: none;
  -moz-appearance: none;
  width: 20px;
  height: 20px;
  background: transparent;
  border: none;
  position: relative;
  cursor: pointer;
}

.boxes input[type="checkbox"]:checked {
  background: transparent;
}

.boxes input[type="checkbox"]:checked::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 1px;
  background: #4a7c59;
  transform-origin: 0 0;
  transform: rotate(45deg) scaleX(1.414);
}

.boxes input[type="checkbox"]:checked::after {
  content: '';
  position: absolute;
  top: 0;
  right: 0;
  width: 100%;
  height: 1px;
  background: #4a7c59;
  transform-origin: 100% 0;
  transform: rotate(-45deg) scaleX(1.414);
}

.boxes{
  display: grid;
  grid-template-columns: repeat(32, 1fr); /* 32 equal columns */
  grid-template-rows: repeat(32, 1fr); /* 32 equal rows */
}