.square {
  width: 100px;
  height: 100px;
  background: #eee;
  font-size: 800%;
  line-height:1em;
  text-align:center;
}

#sudokuboard table, th, td {
  border: 1px solid black;
}

<style>
/* Style The Dropdown Button */
.dropbtn {
  background-color: #4CAF50;
  color: white;
  padding: 16px;
  font-size: 16px;
  border: none;
  cursor: pointer;
}

/* The container <div> - needed to position the dropdown content */
.dropdown {
  position: relative;
  display: inline-block;
}

/* Dropdown Content (Hidden by Default) */
.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}