Here is how you would go about it – if you haven’t figured it out already. You can insert the following CSS into Custom CSS tab in the Theme Options panel or the /custom/custom.css file

#featured-sources {
border: solid 1px #CACACA;
background-color: #E5E5E5; /* Current color in demo site. Set it to something like #c5c5c5 or #d5d5d5 or #eaeaea if you need darker background */
}