Custom Background by Page IDs

The CSS is generated on the file inc/custom-css.php, the background color is set on line 76:

$background = sanitize_hex_color_no_hash( get_theme_mod( 'background_color' ) );


So you can take advantage of the theme_mod_{$name} filter, which changes the value of the get_theme_mod($name) function, by adding this to your functions.php file:

add_filter(
    'theme_mod_background_color',
    function( $value ) {
        $custom_bg_page_ids = array( 18, 19, 20 );
        $custom_bg_color="ca2d2d";
        global $post;
        if ( in_array( $post->ID, $custom_bg_page_ids ) ) {
            return $custom_bg_color;
        }
        return $value;
    }
);