Can we add hovering effect on navItems?
I'll reach out if our team is able to ship this feature! 
For custom css, you have to open the app in end user mode & use the browser tools to find the class. The classes can change without any warning, so it's something you may need to modify in the future. More info on custom css here: https://community.retool.com/t/how-to-write-custom-css-in-retool/2165
