Increase visual contrast of carousel controls

Created by: exige81

Depending on the content of a carousel, the control icons and indicators can get lost against the background, especially when the background content is light.

Changing colors of the controls is not a good solution if you don't know what the background will be in advance (e.g. an image album).

By using the SVG stroke property for the carousel-control-*-icons and using a box-shadow for the carousel-indictator, we can get carousel controls that work well against multiple backgrounds. This would solve some of what was brought up in #21985 (closed)

Codepen of default & enhanced controls

If something like this is a good, idea, I'll be happy to submit a PR.