A smooooooth crossfade change between native tab contents

This quick bit of custom code sets the current tab to display block, any tab that is not current to opacity:0, the active tab to opacity:1 (100%) and sets a transition for the .w-tab-pane's opacity to 1 sec, ease-in.

These changes allow for a crossfade style effect between the content (often images) with the .tab-content and you can do it without changing any class names!