I want to add a toolbar button, but the only way I know how to do that is to drag a button for an existing command from SETTINGS -> CUSTOMIZE -> COMMANDS. I see numerous other icons under SETTINGS -> PREFERENCES -> DISPLAY -> TOOLBAR ICONS. How do I get one of those icons on the toolbar so I can make a command out of it?
Create a new button and then set its icon to whatever you want by editing the button and clicking on the icon in the editor.
To create a new button, enter Customize mode and then right-click an empty space on the toolbar and use the New menu that appears. Or drag things from the New category of the Customize dialog's list of toolbars.