Add CSS

I want to reduce the font size when the width is less than 400px.
(Responsive Breakpoint = 780px)

@media only screen and (max-width: 480px)
#mega-menu-wrap-primary #mega-menu-primary>li.mega-menu-item>a.mega-menu-link {
font-size: 5px;
}

How can I make the above description reflect the CSS?

Regards.
Malibu