#chessboard-container
{
    display: flex;
}
#chessboard-container div span
{
    display: block;
    width: 4em;
    height: 4em;        
}
#chessboard-container div:nth-child(2n+1) span:nth-child(2n)
{
    background-color: lightseagreen;
}
#chessboard-container div:nth-child(2n+1) span:nth-child(2n+1)
{
    background-color: lightblue;
}
#chessboard-container div:nth-child(2n) span:nth-child(2n+1)
{
    background-color: lightseagreen;
}
#chessboard-container div:nth-child(2n) span:nth-child(2n)
{
    background-color: lightblue;
}