如何在simpleHTMLDom中添加body标签中的属性

Okay, so this is the tag i want to modify in my snippet:

  <body style="; width: 100%;" class="portrait" data-promo-button="">

I don't want to change it's innerHTML, but it's style attribute. I want to add the following to the style attribute:

  background-color:black;

it's the only body tag in the whole snippet. I use simpleHTMLDom to get the content, where the body tag is included:

  function game_login($link)
  {
       global $options;
       $ch=curl_init($link);
       curl_setopt_array($ch, $options);
       $content=curl_exec($ch);
       echo $content;
  }

The body tag i want to edit, is stored in the $content variable

Thanks in advance

$string ='    <body style="; width: 100%;" class="portrait" data-promo-button=""> </body>';
    $html = str_get_html($string);
    $html->find('body', 0)->style .= 'background-color:black;';
    echo $html;