exampleTwineCSS.css

@import url(https://fonts.googleapis.com/css?family=Press+Start+2P);

tw-story {	
  /* The following changes the text */
  color: darkgreen; /* Set text to green or other color */
  text-shadow: 1px 1px lightgreen; /* Green text shadow */
  font-size: 20px;
  font-family: "Press Start 2P","Helvetica","Arial",sans-serif;
	
  /* Set background to fade lightblue to blue */
  background: lightblue; /* For old browsers */    
  background: -webkit-linear-gradient(lightblue, blue);
  background: -o-linear-gradient(lightblue, blue);
  background: -moz-linear-gradient(lightblue, blue);
  background: linear-gradient(lightblue, blue);
	
  /* Add a Mars background that will cover the screen */
  background-image: url("http://www.ohiofi.com/img/mars.jpg");
  background-size: cover;
}

tw-passage {
  /* The following changes the text box */
  border: 5px groove gray; /* Gray border around the box */
  border-radius: 25px; /* Rounded corners on the box */
  background: rgba(255,255,255,0.7); /* 30% transparent box */
  padding: 25px;
}

tw-link {
  color:darkblue; /* Set the link color to darkblue */
  transition: color .5s ease-in-out;
}

tw-link:hover { /* Mouse over a link & it turns orange */
  color: orange;
}

img { /* Images won't get wider than tw-passage container */
  max-width: 100%;
}

Thank you for your work! I took your code to tweak my story. But I think, there's been a change in Twine, the background stays black unless you set the background settings in tw-story instead of body.

Thank you for letting me know! Updated to fix the change in Twine