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