Please follow to following process to change the background color of :-
Main background
Goto admin menu >> appearance >> customizer >> Background color >> Main background color
Top menu that shows logo, social link and sidebar nav.
Goto admin menu >>appearance >> customizer >> Background color >> Top menu background color
Middle site title which is above menu that display site title and logo after scroll down.
Goto admin menu >> appearance >> customizer >> Background color >> Middle site title background color
Menu
Goto admin menu >> appearance >> customizer >> Background color >> Menu background color
Category background color which appear in banner and feature display section.
Goto admin menu >> appearance >> customizer >> Background color >> Banner / feature display category background color
Navigation that shows page number in blog post section.
Goto admin menu >> appearance >> customizer >> Background color >> Navigation link background color
Widget title
Goto admin menu >> appearance >> customizer >> Background color >> Widget title background color