After inspecting the developer tools, I noticed that by changing the default li > a background color to red, the background color successfully transitioned to transparent when hovering and clicking on it.
.masthead-nav > li > a:hover,
.masthead-nav > li > a:focus {
background-color: transparent;
}
If you're not seeing any changes, it may be because an initial background color is not set. Simply add something like this:
.masthead-nav > li > a {
background-color: rgba(0,0,0,0.5); /* or choose your desired color */
}