NEVER edit ShiftNav’s core files. Follow the best practices outlined here to customize ShiftNav in a way that will be preserved when you update
Customizer
As of ShiftNav 1.4, Pro customers can customize their menu styles using the Customizer
CSS Tweaks
For minor custom CSS tweaks, you can add them via the ShiftNav Control Panel.
Custom CSS Stylesheet
For more significant custom CSS, you can use a custom stylesheet.