New: multiple css files in a child theme

Hi,

I made a child theme for my parent theme, and importing the main style.css is fine. Bt whenever I try to add anything beneath the main import — for instance, the parent theme also has a black.css that I need to customize and have override all other css for colors — if I add another import url under the main style.css, my whole nav disappears when the modified child style.css is uploaded. It also happens when I try to add any custom css below the import url. What do I need to do so it will work?