When developing web sites it is useful to learn to use either, the Inspector in Firefox, Chrome Developer Tools in Chrome, or one of the other browser developer tools, they will save you hours on things like this and they're very easy to use.
Use this code...
#g-mainbar {
background-color: rgba(255,255,255,0.5);
}
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.