GXT 4.0.x custom split button appearance barebones.

CustomCss3ButtonCellAppearance.java

import com.google.gwt.core.shared.GWT;
import com.sencha.gxt.theme.triton.client.base.button.Css3ButtonCellAppearance;

public class CustomCss3ButtonCellAppearance<C> extends Css3ButtonCellAppearance<C> {

  public interface CustomCss3ButtonResources extends Css3ButtonResources {
    @Override
    @Source({ "com/sencha/gxt/theme/triton/client/base/button/Css3ButtonCell.gss",
        "com/sencha/gxt/theme/triton/client/base/button/Css3ButtonCellToolBar.gss",
        "CustomStyles.gss" }) // override styles here
    Css3ButtonStyle style();

    // TODO extend for overriding the super class assets
  }

  public interface CustomCss3ButtonStyle extends Css3ButtonStyle {
    // TODO override by adding the superclass methods to this interface, with annotation resource path override
  }

  public CustomCss3ButtonCellAppearance() {
    super(GWT.<Css3ButtonResources>create(CustomCss3ButtonResources.class));
  }
}

CustomStyles.gss

/* override the other class selectors in this gss */

gistfile1.txt

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.RootPanel;
import com.sencha.gxt.cell.core.client.SplitButtonCell;
import com.sencha.gxt.widget.core.client.button.SplitButton;
import com.sencha.gxt.widget.core.client.container.FlowLayoutContainer;
import com.sencha.gxt.widget.core.client.menu.Menu;
import com.sencha.gxt.widget.core.client.menu.MenuItem;

public class CustomSplitButtonExample implements EntryPoint {

  private SplitButton button1;

  @Override
  public void onModuleLoad() {
    Menu menu1 = new Menu();
    menu1.add(new MenuItem("Menu 1"));
    menu1.add(new MenuItem("Menu 2"));
    menu1.add(new MenuItem("Menu 3"));
    
    // Override the appearance, the look and behavior
    CustomCss3ButtonCellAppearance<String> customAppearance = new CustomCss3ButtonCellAppearance();
    
    SplitButtonCell cell1 = new SplitButtonCell(customAppearance);
    
    button1 = new SplitButton(cell1, "Button");
    button1.setMenu(menu1);
    
    FlowLayoutContainer flc = new FlowLayoutContainer();
    flc.add(button1);

    RootPanel.get().add(flc);
  }

}

This is how you could override the appearance of the split button.