The slider option is selected when using the section types Slideshow and Full slideshow. This allows a user to scroll right when viewing content. The default color is black and can be very simply changed.
Navigate to your Template and select the EDIT button
Navigate to the Section using the Slider
Select the Pencil icon (edit)
Scroll to the Slideshow UI color and select color
For Full slideshow you have one additional option: Scroll to the Slideshow arrow color and select color that will fill the arrows that appear on the left and right sides in the center. Additionally you can select the color for the Slideshow arrow background color
Tick Save and this will be applied to all content in this section