Hello,
This background is an echo based on the highlight color used in the Template Manager. If you want to remove it you would need to navigate inside the index.php file and remove the call that applies this echo to the Horizontal Accordion. This is the default coding output for your site:
.horzaccordion li:hover .image-title {
background: none repeat scroll 0 0 rgba (33, 159, 255, 0.9) !important;}
But the background section in the index.php will most likely be an echo of the s5_highlight color.