// .h

MyApp : public DockWindow {

    ...

    static void SetTabCtrlStyle();

    ...
};

//  .cpp

void MyApp::SetTabCtrlStyle()
{
    TabCtrl::Style& tab_style = TabCtrl::StyleDefault().Write();
    
    // Set your TabCtrl style here
    tab_style...

}

GUI_APP_MAIN
{
    // set CtrlTab style BEFORE creating main app window
    MyApp::SetTabCtrlStyle();

    MyApp().Run();
}