Before adding any custom CSS, it's recommended to give the slider that you want to target a custom class. This can be set in the Custom Class field, in the slider's Appearance sidebar slide.
Note: The custom class name you specify in the Custom Class field doesn't need to include the dot, only the name of the class. For example, you need to enter my-slider , not .my-slider .
After you assigned a custom class to the slider, you can use it in the Custom CSS field, to target that specific slider.
You could use a general selector, like .slider-pro if you want to target all your slider instances.