Sure, Use this code...
.mrt-bgvideo-wrapper .mrt-bgvideo-container:before {
content: "";
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
background-color: rgba(0,0,0,0.5);
}
adjust alpha (0.5) to suit your needs
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.