Take a look in the Index.java file. Most of a GWT application is added by
code in the Java files and won't show up in the HTML.
If you want to add CSS to hide the mainHeader DIV that would look like this:
#mainHeader {
display: none;
}
You can also remove the MainHeader from the Java code.
Spiffy UI is a framework based on GWT which has you write Java code to
generate your web page. I would recommend you find a book on Java
programming and a book on web development to help you navigate the basic
concepts of GWT programming.