If you want to add a significant amount of custom CSS, the best method is to use a custom stylesheet.
You can always add the CSS to your child theme’s style.css, but if you want to keep your ShiftNav styles separate, you can use the ShiftNav custom.css
1. Activate the custom stylesheet
In the ShiftNav Control Panel, change your Skin setting to Custom (custom.css)
This will load the custom/custom.css file after the ShiftNav core files, but before the Custom Tweaks from the settings.
2.Create the custom stylesheet
In the plugins/shiftnav-pro/custom directory, create a custom.css file (you can rename custom-sample.css).
3. Adding skin and supplementary styles
It’s easiest to begin by using the contents of the custom-sample.css skin and changing or adding styles as you like.
4. Using LESS
If you are familiar with LESS, you can use the custom-pro-sample.less file to generate your custom skin.
Have fun customizing!