Theme: Twenty Twelve

Have you done any modifications to the theme stylesheet at all? If so, move the changes over to Custom CSS plugin and keep theme stylesheet as new.
https://www.ads-software.com/plugins/search.php?q=custom+css

Install a Custom CSS plugin and try this code. Change color and size to your liking.

.site-content article {
	border-bottom: 4px solid #910091;
}
article.sticky .featured-post {
	border-top: 4px solid #910091;
	border-bottom: 4px solid #910091;
}

The above solution should help you.