zero-blog/views/admin_stylesheet.scss

#messages {
  .message {
    border: 1px solid black;
  }
  .error {
    @extend .message;
    background-color: red;
  }
  .warning {
    @extend .message;
    background-color: orange;
  }
  .notice {
    @extend .message;
    background-color: green;
  }
}