Not quite sure what the OPs intention is. If it is just to apply both sets of styles to a single element then <div id="container" class="reusable"> would be fine.
If you want to apply everything contained within 'reusable' but only that contained within 'container' to your container div then you would switch what you have slightly to:
#container, .reusable {
float:left;
width:178px;
padding:8px;
margin:0 3px 0 0;
}
#container {
background: url(../images/sb.jpg) no-repeat top left;
background-color:#e0f2ff;
}
If you mean something else then explain your question better
