See Twine here: http://kennethsherwood.com/twines/sample-font-changes.html

Copy code and go to “edit story stylesheet”

Obviously, select your own colors and font sizes / styles. Note, the font you choose should be a common Web Safe Font.

body, tw-story
{
font-family: Palatino;
font-size: 48px;
color:red;
background-color:green;	
}
tw-passage {
width: 90%; padding: 2em; 
  margin-top: 10px;
  margin-bottom: 10px;
  margin-right: 10px;
  margin-left: 10px; 
 background-color:blue;
}