You will need to adjust the widths this way to achieve this. Pls check if the responsive layout is ok after this change input in Custom CSS tab in theme options or the custom/custom.css file-
#content {
width: 680px; /* Increased by 40px */
}
.sidebar-right-nav {
width: 240px; /* Decreased by 40px; */
}
.layout-2c .thumbnail-list .image-container, .layout-3c .thumbnail-list .image-container {
width: 220px; /* Increased by 10px */
}
.layout-2c .thumbnail-list .entry-text-wrap {
width: 436px;/* Increased by 30px */
}