Gotcha. The CSS above is intended to move the drop-down menus over a bit so they aren’t cut off on the right. If you’re wanting a separate menu system for mobile, we’ll want to use a plugin here. I would suggest trying the TinyNav plugin (https://wordpress.org/plugins/tinynav/). It replaces WordPress’ default menu system with a select one on smaller screens that’s easier to use.
Once you’ve installed and activated the plugin, there will be a new settings screen in the settings section of your WordPress dashboard. Here is what I would suggest adding there.
In the Custom ID/Class field, use .menu
In the screen size field, I would use 767 as the size where to apply this.
Next add this to your Custom CSS to add a bit of style to it: