You can use the custom css section in the ACP to customize it, just give the navigation items a custom class in the class field. Then use css to target that.
What does it depend on if the icons are displayed in the theme?
Forgive me for this question, but I'm learning. How do I fix the non-functioning category icons in the template?
What do you mean by non functioning?
I'm currently using Slick theme and the section icons are not displaying. How can I fix this ?