Your sidebar has its position set to absolute. When it's absolute, the parent container doesn't stretch to contain it. This is so that you can position it wherever you want without affecting any other elements. Here are the CSS overrides you need (to override existing CSS, put this at the end after all of the other styles) so that the parent container stretches with the sidebar.
#main {
overflow: hidden; /* needed to stretch parent container since children are floated */
}
#primary.content-area {
width: 68%;
float: left;
}
.site-main .sidebar-container {
position: static;
float: right;
width: 30%;
height: auto;
}
.sidebar .entry-header, .sidebar .entry-content, .sidebar .entry-summary, .sidebar .entry-meta {
padding: 0;
}