If you have made a total mess of your Toolbar like me, there's no button for returning the icons to the default set, but getting them back there is pretty easy. First, Shift-click on the white pill-shaped button at the top right of your window's title bar to bring up the Customize Toolbar dialog. In the bottom left of the Customize Toolbar dialog, you'll see a set called "the default set." Drag it up top and it will replace the current icons in your Toolbar.