The Extra Theme doesn’t (yet) include an option to set the background color on the titles of widget boxes in the custom sidebar widgets and sidebar module. Fortunately, it is easily done with this bit of CSS:
/* === Change background color on widget titles === */
/* */
.et_pb_widget .widgettitle, 
.et_pb_widget .wp-block-group__inner-container > h2, 
.et_pb_widget .wp-block-group__inner-container > h3 {
    background:#D0C2B2 !important;
/* === END === */

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