Getting the ship to text in the invoice LARGER

need some help over here... !

I am trying to make the ship to text larger on the invoice, so I am in the process of creating a DIV section, but can´t seem to make it work. So far, i have done the following:

Inside the invoice.php at the admin:

<div id="invoicefont">
            <td class="main"><b><?php echo ENTRY_SHIP_TO; ?></b></td>
          </tr>
          <tr>
            <td class="main"><?php echo zen_address_format($order->delivery['format_id'], $order->delivery, 1, '', '<br>'); ?></td>
          </tr>
          <tr>
            <td class="main">Tel: <?php echo $order->customer['telephone']; ?></td>
          </tr>
<tr>
            <td class="main"><?php echo '<a href="mailto:' . $order->customer['email_address'] . '">' . $order->customer['email_address'] . '</a>'; ?></td>
          </tr>
</div>

and inside my /public_html/includes/templates/(Template custom)/css/stylesheet.css at the very bottom:

#invoicefont {

    font-size: 10.9em;

    }

but i believe i might be lost...

anyone please?

or a link tutorial?