How can I add borders or shadows around the Carousel slider type's slides?

Having borders around the  carousel slider type is kind of problematic, as this changes the slide's size and the responsive behaviour (how the slides are disappearing) and the slide switching is based on this size too. The single switch option creates some issues, so you will see the options you have without the single switch and with that option.

First check out what your  slider's ID is, because these codes only will be applied to one given slider. In my example my slider's ID will be: 15
So replace every 15 number with your ID number in the following codes.

You can insert these CSS codes for example into your slider's  Developer tab - CSS field.

Without single switch

Border

div#n2-ss-15 .n2-ss-slide{
	border:10px solid white;
	box-sizing:border-box!important;
}

Box shadow

div#n2-ss-15 .n2-ss-slide{
	box-shadow: 0px 0px 10px 2px #7f7f7f;
}

With single switch

With single switch you cannot use border. This can be replaced with outline, and that would work fine, but the limitation you have with this is, that the border radius cannot make outlines rounded. So if you want rounded borders, you will have to turn off the single switch option.

Outline

div#n2-ss-15 .n2-ss-slide{
	outline: 10px solid white;
}
div#n2-ss-15 .n2-ss-slider-pane-single{
	overflow-x: visible!important;
}

Limitation: You cannot use Maximum pane width, because the overflow hides the parts, which suppose to be hidden outside this maximum width.

Box shadow

div#n2-ss-15 .n2-ss-slide{
	box-shadow: 0px 0px 10px 2px #7f7f7f;
}
div#n2-ss-15 .n2-ss-slider-pane-single{
	overflow-x: visible!important;
}

Limitation: You cannot use Maximum pane width, because the overflow hides the parts, which suppose to be hidden outside this maximum width.