The site info does not have a background color option. It is transparent, taking on the background color of the box element. See how to change it here.

http://knowledgebase.angiemakes.com/how-to-change-box-background-color/

You could also just apply some custom CSS to add a background color to the site info.

#master .site-info {
  background-color: #fcf8f7;
}