I see that the Tree Node widget gives the following styling to the element, which causes a margin of 8px on the left side of your text. In order to override this, you have to apply some custom styling.
Create your own CSS class in your custom stylesheet within your project and apply the following properties to it:
Open your Tree Node widget, go to the 'Appearance' tab and apply the 'treenodenomargin' as your Class. Note that 'treenodenomargin' was just an example, you can obviously come up with a better name.