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