GXT 4 custom styles for the tab bar. Using the client bundle css resources.

styles.gss

.bar {
}

.bar span > span {
    color: red !important;
}

TabPanelResources.java

import com.google.gwt.core.shared.GWT;
import com.google.gwt.resources.client.ClientBundle;
import com.google.gwt.resources.client.CssResource;

public interface TabPanelResources extends ClientBundle {

  public static TabPanelResources INSTANCE = GWT.create(TabPanelResources.class);

  public interface Styles extends CssResource {
    String bar();
  }

  @Source({ "styles.gss" })
  Styles styles();

}

TabPanelWithCustomStyles.java

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.user.client.ui.HTML;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.Widget;
import com.sencha.gxt.widget.core.client.TabItemConfig;
import com.sencha.gxt.widget.core.client.TabPanel;
import com.sencha.gxt.widget.core.client.container.Viewport;

public class TabPanelWithCustomStyles implements EntryPoint {

  @Override
  public void onModuleLoad() {
    TabPanelResources.INSTANCE.styles().ensureInjected();
    
    Viewport vp = new Viewport();
    vp.add(createTabPanel());
    
    RootPanel.get().add(vp);
  }

  private Widget createTabPanel() {
    TabItemConfig config1 = new TabItemConfig();
    config1.setText("Tab 1");
    config1.setClosable(true);

    TabItemConfig config2 = new TabItemConfig();
    config2.setText("Tab 2");
    config2.setClosable(true);

    TabItemConfig config3 = new TabItemConfig();
    config3.setText("Tab 3");
    config3.setClosable(true);

    TabItemConfig config4 = new TabItemConfig();
    config4.setText("Tab 4");
    config4.setClosable(true);

    TabPanel tabs = new TabPanel();
    tabs.addStyleName(TabPanelResources.INSTANCE.styles().bar());
    tabs.add(new HTML("Tab 1"), config1);
    tabs.add(new HTML("Tab 2"), config2);
    tabs.add(new HTML("Tab 3"), config3);
    tabs.add(new HTML("Tab 4"), config4);

    GWT.log("bar: " + TabPanelResources.INSTANCE.styles().bar());
    
    return tabs;
  }

}