What does #document mean? And how can I select elements outside of that code?

What does #document mean?

I have a webpage built by a tool kit. There is a widget whit that I can embed own code. But it is encapsulated as an extra #document within the general html code of the page. I would like to overwrite the css of the general webpage, maybe using this embed code widget. How can I get elements (parents) outside of this extra #document? Thanks a lot for any help!

Miminal example:

 <html>
  <head>...</head>
  <body>
    <div class="title">Hello</div>
    ...
    #document
      <html>
        <head>...</head>
        <body>
          i would like to inject anything here to change the color of the title
        </body>
      </html>
  </body>
</html>