diffrent style for post single page

You could listen for the $_SERVER['HTTP_REFERER'] and add a class to the body depending on that referer

add_filter( 'body_class','wpse_body_classes' );
function wpse_body_classes( $classes ) { 

  if ( is_page( 'event' ) ){ // event is the page slug of the page we wish to add the class for


    if( wp_get_referer() == '/url/of/get_involved' ){

      $classes[] = 'my-light-bg';

    }

    elseif( wp_get_referer() == '/url/of/urban_innovation_center' ){

      $classes[] = 'my-dark-bg';

    }

    else {

      $classes[] = 'my-default-bg';

    }    

  }

  return $classes;

}

EDIT

Updated the code to

• make use of wp_get_referer
• check if we are on the event page so the filter only runs on that page
• add a default body class to that page if no referrer is set