I just figured out how to do this.
Essentially, line numbers are assigned a style, just like almost any other special formating in Word.
Process
- Go to the Home tab of the Ribbon after enabling line numbering.
- Under Styles, click the Dialog Box Launcher arrow to open the Styles pane.
- In the Styles pane, click Options.
- Under "Select styles to show" in the Options window, select "All styles". Under "Select how list is sorted", select "Alphabetical"; this option makes it easier to locate the style in question.
- Click OK to exit out of the Options window.
- Back in the Styles pane, scroll down until you find the Style that is called "Line Number". Click the arrow next to it, hit Modify, and change formatting as necessary in the window that opens. Click OK once you're finished and then close the Styles pane.