Use this code...
.fp-header .fp-header-slideshow .g-owlcarousel-layout-standard .owl-item > div .image::before {
background-image: none
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.