The newly launched Extra Theme doesn’t (yet) include an option to set the background color on the titles of widget boxes in the sidebar. Fortunately, it is easily done with this bit of CSS (provided by ET on their forum):

/* Change widget title colors */
.et_pb_extra_column_sidebar .et_pb_widget .widgettitle { background:#D0C2B2; }

You can add this to the theme by pasting it into the “Custom CSS” box found in the ePanel at “Extra > Theme Options > General Settings” (at the very bottom). Just change the #D0C2B2 part to the hex value of your desired color.