Please follow to following process to change the background color of :-
Main background
Goto admin menu >> appearance >> customizer >> Background color >> Main background color
Menu background color
Goto admin menu >>appearance >> customizer >> Background color >> menu background color
Category background color which appear in feature display section.
Goto admin menu >> appearance >> customizer >> Background color >> 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