How to load .css file only on your custom 404 Not Found page?
As mentioned by @Yogesh, you should use drupal_add_css. Your code cannot work because you have missing slash. Please use below code.

function themename_preprocess_page(&amp;$vars) {    
 // Adds a Custom 404 page    
 $header = drupal_get_http_header("status");    
 if($header == "404 Not Found") {        
   $vars['theme_hook_suggestions'][] = 'page__404';
   drupal_add_css(drupal_get_path('theme', 'dev') . "/css/404/bootstrap.css");
   drupal_add_css(drupal_get_path('theme', 'dev') . "/css/404/custom-style.css");
 }    
}