button  { width:22px; height:22px; background-color:white; border-width:1px; }
.given  { color:black;                     border-color:black;                             }
.null   { color:white; border-style:none;                                                  }
.sink   { color:red;       border-color:red;                                           }
.source { color:red;       border-color:red;                                           }
#space  { display:none;  }
#letter { color:black;     border-color:black; width:72px; height:72px; border-width:2px; display:none; }
#number { color:black;     border-color:black; width:72px; height:72px; border-width:2px;               }
.sink:hover,  .source:hover  {              background-color:yellow; }
.sink:active, .source:active { color:white; background-color:red;    }