Make cell borders gray instead of black

src/index.css

 }
 
 .cell {
-  border: 1px solid black;
+  border: 1px solid #999;
   border-right: none;
   border-bottom: none;
   display: inline-block;
 }
 
 .row:last-child .cell {
-  border-bottom: 1px solid black;
+  border-bottom: 1px solid #999;
 }
 
 .cell:last-child {
-  border-right: 1px solid black;
+  border-right: 1px solid #999;
 }
 
 .cell.alive {