Mobile menu appears blank, failing to change styles

At some point in developing my site, the mobile menu stopped displaying any of my navigation.

I tried to make the nav show up by changing the stylesheet e.g. color of text, z-index, background color etc but none of my changes have shown up. (see below)

I may be reading the dev tools incorrectly, missing a style that could help, or there could be something more complex afoot.

@media screen and (max-width: 600px) {

#mobile-navigation-jquery {
  background-color:rgb(28, 34, 48)!important;
  z-index:200;
}

#menu-header,
#menu-item-49,
#menu-item-442,
#menu-item-56,
#menu-item-54,
#menu-item-51,
#menu-item-667 {
  z-index:200;
  color: black;}

}