I would advise against using a class altogether and instead use a nested list. It makes more sense semantically. Just target ".list_container ul ul" to get the entire sub list.
Code:
<ul>
<li><a href="index.php">Hjem</a></li>
<li><a href="Marmeladeby/Marmeladeby.php">Marmeladeby</a></li>
<li><a href="Event/Event.php">Event</a></li>
<ul>
<li class="2"><a href="Teatersport.php">Teatersport</a></li>
<li class="2"><a href="Juletrefest.php">Juletrefest</a></li>
<li class="2"><a href="Grand_Prix_Show.php">Grand Prix Show</a></li>
</ul>
<li><a href="Kurs.php">Kurs</a></li>
<li><a href="Om_oss/Om_oss.php">Om oss </a> </li>
<li><a href="Om_oss/Kontakt.php">Kontakt</a></li>
</ul>
Also, you would do good to validate your document. It contains a whole bunch of errors right now and I think most could be avoided by correcting just one of them, the misplaced <body> tag at the bottom of the document.