// set the modern theme to visualize the summary better
final ThemeConfiguration defaultTheme = new DefaultNatTableThemeConfiguration();
defaultTheme.addThemeExtension(new DefaultGroupByThemeExtension());
final ThemeConfiguration modernTheme = new ModernNatTableThemeConfiguration();
modernTheme.addThemeExtension(new ModernGroupByThemeExtension());
final ThemeConfiguration darkTheme = new DarkNatTableThemeConfiguration();
darkTheme.addThemeExtension(new DarkGroupByThemeExtension());
natTable.setTheme(modernTheme);