webextensions-examples/page-styles/styles.css

#output {
  border: 1px solid blue;
  padding: 1em;
  margin: 1em;
  clear: both;
}

#wrap {
   width:600px;
   margin:0 auto;
}
#left-column {
   float:left;
   width:300px;
}
#right-column {
   float:right;
   width:300px;
}

input {
  margin: 0.5em;
}