Thanks Pavel,
I've been using the inspector frequently for my adjustments.
I found that code. I tried changing the value for background and clearing the cache, and refreshing the browser and that did not work.
update...
I tried adding this code to the custom css Temple Custom Code panel and that worked. Odd that it didn't work in the actuall css file.
.sppb-slider-fullwidth-wrapper .sppb-slider-item-wrapper .sppb-slideshow-fullwidth-item .sppb-slideshow-fullwidth-item-bg:after {background: rgba(28,49,74,0.3);}
Thanks Pavel for verifying this is the active code responsible for this property.