.site-info
{
  clear:both;
}

#supplementary {
	border-top: 1px solid #ddd;
	padding: 1.625em 7.6%;
	overflow: hidden;
}

/* Two Footer Widget Areas */
#supplementary.two .widget-area {
	float: left;
	margin-right: 3.7%;
	width: 48.1%;
}
#supplementary.two .widget-area + .widget-area {
	margin-right: 0;
}

/* Three Footer Widget Areas */
#supplementary.three .widget-area {
	float: left;
	margin-right: 3.7%;
	width: 30.85%;
}
#supplementary.three .widget-area + .widget-area + .widget-area {
	margin-right: 0;
}

How to put sidebars footer on twenty twelve theme again. Part 5: add the styles necesary in style.css (this styles are no optimized -yet- for the responsive layout)

WP twentytwelve theme width sidebar footer again. Part 5: STYLE.CSS

wordpress

CSS