What a test should i do in FullStack React Express App?

I am building a FullStack Weather App with React and Express. I am gonna try to say a few words about this app. So basically as I mentioned my frontend side is React, here I am using hooks and to styled everything, I decided to use styled-components. For my backend, I am using Express with MongoDB and mongoose. I have a sign-up and login page in my app where I do Axios to communicate with express to sign up or log in as a user. When a user login correctly then I load the main page where a user can check the weather by typing city name into a form. Also, I am gonna use Redux and JWT but this, not a question, this is only to illustrate my app to you. I decided to add testing to my app with JEST and this is my question. I am wondering what should I test with styled-components? What should I test in my app? Functions?