The Carousel Style tab allows you to adjust the background color, stroke (outline), and margin around the Carousel.
The background of the Carousel is the color that appears behind the thumbnails. If you're using background images they will cover up the background color everywhere except where the images are transparent.
Stroke is just a fancy name for the Border. You can add a border of any color and thickness you like around all or none of the Carousel.
The Carousel margin is the space that is added between other items on the page and the outside of the border of the Carousel.