style.scss

$primary-color: #88f;
$empty-color: #ddd;
$border-color: #444;

.cell {
  fill: $empty-color;
  stroke: $border-color;
  cursor: pointer;

  &:hover {
    fill: darken($empty-color, 10%);  
  }

  &.alive {
    fill: $primary-color;
  }
  &.alive:hover {
    fill: darken($primary-color, 10%);
  }
}