That's because the monobook template uses ids named "content" to format parts of the document. See
lib/tpl/monobook/monobook/main.css, Line 18:
#content {
margin: 2.8em 0 0 12.2em;
padding: 0 1em 1.5em 1em;
background: white;
color: black;
border: 1px solid #aaa;
border-right: none;
line-height: 1.5em;
position: relative;
z-index: 2;
}
A quick hack could be:
1.) Change the above part to
div.content {
margin: 2.8em 0 0 12.2em;
padding: 0 1em 1.5em 1em;
background: white;
color: black;
border: 1px solid #aaa;
border-right: none;
line-height: 1.5em;
position: relative;
z-index: 2;
}
2.) Open the file
lib/tpl/monobook/main.php, Line 57:
<div id="content">
Change that to
<div class="content">
But monobook is a complex layout, that may affect other parts I'm not aware of at the moment. At least now you got an idea what's the reason for this behavior. Good luck ;).