Before adding any custom CSS, it's recommended to give the accordion that you want to target a custom class. This can be set in the Custom Class field, in the accordion's Appearance sidebar panel.
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-accordion , not .my-accordion .
After you assigned a custom class to the accordion, you can use it in the Custom CSS field, to target that specific accordion.
You could use a general selector, like .accordion-slider if you want to target all your accordion instances.