What is Theme module in GWT?

Module	Logical Name	Module Definition	Contents
Chrome	com.google.gwt.user.theme.chrome.Chrome	Chrome.gwt.xml	Style sheet and images for the Chrome theme.
Dark	com.google.gwt.user.theme.dark.Dark	Dark.gwt.xml	Style sheet and images for the Dark theme.
Standard	com.google.gwt.user.theme.standard.Standard	Standard.gwt.xml	Style sheet and images for the Standard theme.

GWT also provides several theme modules which contain default styles for widgets and panels. You can specify one theme in your project's module XML file to use as a starting point for styling your application, but you are not required to use any of them.