Board Thread:Support Requests - Getting Started/@comment-4594870-20140803111314/@comment-5558012-20140803154605

It seems to me that the background-color (which is currently black) is overriding the background transparency. You could try removing the background completely with

.wikiaPhotoGallery-slider-body { background: none !important; }

Then crop the white space on each side of and it should give the same view as using transparent.

As for why you need to import. Common.css only applies to the Monobook skin by default while Wikia.css applies to the Oasis skin. Importing them together allows the styling on Common.css to also be applied to the Oasis skin.