theme Online Shop background color.

Hello, I was attempting to use your theme Online Shop for one of our offshoot websites but needed to change the primary navigation’s background color. I can’t get any css to work on it. . . I was curious what I was doing wrong. Or do I need to go into the style-sheet to change it? I tried adding custom css to the additional css section in the customize menu but no changes happened.

.main-navigation, .main-navigation ul ul {
background-color: #ff0000;
}