It's indeed a bit tricky to get the names/paths of the built-in UI elements... for this reason we've added a small tool which can be enabled by typing uidebugger into console - it shows the path of the currently hovered element in the top left corner of the screen 
While this tool is active, you can also rightclick on an element to copy the UI path to clipboard. To close the tool, just hit the X in the top left corner (if you can't hit it properly, press ESC and try to hit it again).
This enables you to get the paths of most elements
For instance, the path to the inventory background would look like this: InventoryLayer/inventoryContainer etc.
Unfortunately you can't access all elements this way. To overcome this limitation, you could also type uidebugger <layername> into console - this prints the paths of all elements on a particular layer into console.
But if there is a particular UI element you're looking for, just let me know and I can post the path here 