Hi,

You can do that by add more this code to Customize > Custom CSS:

.pencislider-container .pencislider-content .pencislider-title span,
.pencislider-container .pencislider-content .pencislider-caption span{
background-color: rgba( 0, 0, 0, 0.4 );
}

Note Important:

Change values:   0, 0, 0   on the above code to R, G, B values of color you want to chagne for.

You can get R, G, B values for color you want to change on this site - check this image for more.

Best Regards,

PenciDesign