If you wanna custom by CSS, JS, please follow here
Make sure You went to Theme settings > Custom CSS/JS > Enable this section
Please remember to check in Enable Custom CSS so it will apply all code you fill in Custom CSS field
In this field, we already split to some responsive devices like Global, desktop, mobile, tablet, mobile. You can write your code to the correct field. For example, if you want CSS only apply on Mobile so let's add to Custom CSS for only mobile.
You have to Check on Enable Custom JS and go to Edit theme > Custom.js and write your code here