function setWidgetStyle( aWidget, aStyleName )

http://www.google.fr/support/forum/p/apps-script/thread?tid=71e749576207a8a8&hl=en
My solution to this is the following:

var styles = { "header": { "fontSize": "24px", "color": "red" } };

function doGet( e ) {
  var app = UiApp.createApplication();
  var mainPanel = app.createVerticalPanel();
  var mainHeader = app.createLabel( "Hello world!" );
  setWidgetStyle( mainHeader, "header" );
  mainPanel.add( mainHeader );
  app.add( mainPanel );
  return app;
}


function setWidgetStyle( aWidget, aStyleName ) {
  for( attribute in styles[aStyleName] ) {
    aWidget.setStyleAttribute( attribute, styles[aStyleName][attribute] );
  }
  
  return aWidget;
}