Showcase slider type - Dark overlay

You can create the a dark overlay effect by following the steps below.

Switch to Expert mode before proceeding.

Adding the background color
Add a background color to the Slider settings → General tab → under Showcase slider type - Settings: Slider → CSS. E.g.
background-color: #181e24;
Add an opacity to the slides

Scroll further down the page and find the Opacity setting under Showcase slider type - Animation. Turn it on and adjust the Before and After values.