:root {
               background-color: white;
           }
           
           table {
               -webkit-box-shadow: 0px 0px 21px 0px rgba(166, 166, 166, 1);
               -moz-box-shadow: 0px 0px 21px 0px rgba(166, 166, 166, 1);
               box-shadow: 0px 0px 21px 0px rgba(166, 166, 166, 1);
               border: darkgreen 10px double;
               margin: auto;
               text-align: center;
               font-size: 2em;
           }
           
           table td {
               width: 50px;
               height: 50px;
               border: 0px;
           }
           /* odd y even (impar y par) */
           
           table tr:nth-child(odd) td:nth-child(even),
           table tr:nth-child(even) td:nth-child(odd) {
               background-color: darkslategray;
           }
           
           span:hover {
               color: crimson;
           }