Ciao crilla,
aggiungi i margin-left:auto; e margin-right:auto; qui:
#block-main { background-color: #fff; border-top-color: #1d1d1d; border-top-style: solid; border-top-width: 1px; margin-left: auto; margin-right: auto; text-align: center; width: 75%;}
custom.css riga 10