as you are using a commercial theme by elegantthemes, please contact theme for support with your question.
generally, the widget titles have the style:
#sidebar h3.sidebar-title { font-size: 18px; color: #ffffff; font-weight: bold; background: #b7b7b7; border-top: 1px solid #9d9d9d; border-left: 1px solid #9d9d9d; border-bottom: 1px solid #ffffff; border-right: 1px solid #ffffff; padding: 5px 12px; display: inline-block; margin-bottom: 15px; }
which is where you would need to edit the background css; http://www.w3schools.com/css/css_background.asp
using a tool such as Firebug http://getfirebug.com/ might generally help to identify which styles you need to edit for which element.