html, body {
  width: 100%;
  height: 100%;
  font-family: Verdana, Arial;
}
body {
  margin: 0;
  background: #f0f0f0;
}
.box {
  margin: 1em 0;
  width: 100%;
  display:flex;
  align-items: center;
  justify-content: center;
}

#sudoku_board {
    font-size: 5vmin;
    border-top: 1vmin solid #444;
    border-left: 1vmin solid #444;
    border-collapse: collapse;
}

#sudoku_board tr {
    border-bottom: 0.2vmin solid #444;
    border-right: 1vmin solid #444;
}

.sudoku_row_separator {
    border-bottom: 1vmin solid #444 !important;
}

#sudoku_board td {
    width: 9vmin;
    height: 9vmin;
    border-right: 0.2vmin solid #444;
}

.cell_separator {
    border-right: 1vmin solid #444 !important;
}

#sudoku_board {
  background: #fff;
}
#sudoku_board td {
  overflow: hidden;
  text-align: center;
  transition: all 0.25s;
}
#sudoku_board td:focus {
  background: rgba(0, 0, 255, .1);
  outline: none;
}