Adding theme option values as custom body class

Review how the variable scope works..

Let’s not add yet another global variable, instead we can e.g. fetch the option values within the filter’s callback:

function wpse251261_custom_body_classes( $classes ) {

     // Get option values
     $rounded_corner_radio = of_get_option( 'rounded_corner_radio' ); 
     $gradient_radio       = of_get_option( 'gradient_radio' );

     // Assign new body classes
     $classes[] = esc_attr( $rounded_corner_radio );
     $classes[] = esc_attr( $gradient_radio );

     return $classes;
}

add_filter( 'body_class','wpse251261_custom_body_classes' );

Note that we escaped it with esc_attr(), because the body_class filter is applied after such escaping in get_body_class().