OliveWP theme allows to change the background image and background color using the below options. Before displaying the background image and color you need to enable the boxed layout option.

Location: Appearance >> Customize >> Colors & Background >> Background Image

1. Background Image

In this option, you can add the background image with image position and image size options.

2. Background Color

In this option, you can add the background color.

Location: Appearance >> Customize >> Colors & Background >> Background Color

For additional features: Manage the Background Image (Pro)