By appearance, if you mean color theme, you can change or remove the default theme in your PROJECTNAME.gwt.xml
file. Changing/removing this will respectively change/remove the CSS stylesheet that gets automatically attached to your application.
The default is:
<inherits name='com.google.gwt.user.theme.clean.Clean'/>
You can change it to one of the following themes:
<inherits name='com.google.gwt.user.theme.standard.Standard'/>
<inherits name='com.google.gwt.user.theme.chrome.Chrome'/>
<inherits name='com.google.gwt.user.theme.dark.Dark'/>
Or remove / comment those lines altogether.