Return value of get_background_color

To add custom background support in your theme you need to register the default custom background support.

Example:-

add_action('after_setup_theme', function(){
    add_theme_support( 'custom-background', array( 
        'default-color' => 'black' //Default color 
    )); 
});

Now you will see a new section Background Image in customizer and in Color you will see a new option Background color

Once you will save the settings you can use echo get_background_color();