How to have an image span whole page in tumblr theme?

As the title says, I'm wondering how to get this image to span the whole browser window/page instead of having the gray margins.

Im using this theme: https://raw.githubusercontent.com/olleota/themes/master/paperweight/main.html

And, its in action here: http://fvcking5hit.tumblr.com/ where you see the gray borders and I just want it to span the whole page wile keeping the fade effect.

Solution

OK thank you for the additional comment, this should be all the code you need.

Find this code in your theme (it's showing at around line 226):

#masthead {
    margin: 0 10%;
    padding: 15% 0;
    width: 80%;
}

Change it to:

#masthead {
    margin: 0;
    padding: 15% 0;
    width:100%;
}

There are several references to #masthead in your css, so you will need to target the last reference for this to work (or tidy up the css by removing the old properties).

UPDATE

As you pointed out there is a media query changing the properties of the masthead.

Find this code:

@media (min-width: 1500px){
  .theme-name-Paperweight #masthead {
     margin: 0 20%;
     width: 60%;
     padding: 10% 0;
  }
}

And change to:

@media (min-width: 1500px) {
  .theme-name-Paperweight #masthead {
     margin: 0;
     width: 100%;
     padding: 10% 0;
  }
}