Hello

Thank you for your query.

You'll get overlay option (Background Color Overlay) - if you select a slider item (to edit). Kindly check the screensnot below

Hi, this setting is unavailable on my 'Spectrum' quickstart install (localhost) https://prnt.sc/1zo9bgf

Slidershow addon used by the quickstart does not have the overlay effect. There is another addon for slideshow - which have the overlay effect.