Problem with “Undeclared entity warning”

I’ve been setting up a blog for the Asian Art Museum in San Francisco (where I work). At the very bottom of the page, below the footer, I am seeing an error message that says “Undeclared entity warning at line 282, column 1.” I’ve searched for some offending character but haven’t turned up the problem.

The theme I am using comes with a few different styles. I believe the problem is related to the supplemental style sheet for the flavor I am using (called “grey”), but I don’t see what’s wrong with it. (Something about the background image maybe?) I will append the css at the end of this post.

You can see the problem here: http://www.asianart.org/blog/

Can anyone help?
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Here’s “grey.css”:

body {
background-image: url(http://www.asianart.org/blog/background-images/zhang-daqian.jpg); background-repeat: repeat
}
#menu ul {
background: url(grey/menu_bg.gif) repeat-x 0 bottom;
}
#menu ul li a {
border-left:1px solid #3a4c2b;
}
#search {
background: url(grey/search_block_bg.gif) no-repeat 0 0;
}
#container {
background: url(grey/top.gif) no-repeat 0 143px;
}
.background {
background: #F5F5F5 url(grey/content_bg.gif) repeat-y right top;
}
#sidebar .block, #sidebar .adverts{
background: url(grey/sidebar_block_top.gif) repeat-x 0 0;
}
#footer {
background: url(grey/footer_bg.gif);
}
#footer .left-col h2 {
background: url(grey/footer_element.gif) no-repeat right 18px;
}
#footer .left-col ul li {
background: url(grey/footer_line.gif) repeat-x left bottom;
}
#copyright {
background: #2b2b2b url(grey/bottom.gif) no-repeat left bottom;
border-top: #424242 1px solid;
color: #80807f;
}

.post-content a span.read_more {
color: #97c0f0;
}

#footer a {
color: #90938e;
}

#footer a:hover {
color: #a6a9a4;
}

#footer h2 {
color: #848783;
}
#sidebar h2 {
color:#91938f;
}
#sidebar .block a {
color:#676966;
}
#wp-calendar th{
background:#dfe0df none repeat scroll 0 0;
}
#footer #subscribe {
color:#e1e2e3;
}