how to change the css of nav-menu and change the background of basic

[郭志鹏]

I am a user of superset , I think superset is a very good data exploration platform designed to be visual,  I want to change the UI to show my date now , but I find this nav-menu is built use appbuilder/menu.html that is create by appbuilder so I can't use css and html change this UI directly ,and  I want to change this background images but I can't . so I want to how I should to do ,and can you give me some suggestions ?