Thank you for reaching out to is. Setting up a child theme allows you to make code changes that won’t be overwritten when an X update is released. So it's highly recommended to install a child theme and move all of your customization to your child theme. If you've only added custom CSS then that would be easy, simply copy all of your custom CSS and paste it into child theme's style.css file.
Even at this point you can safely install the child theme and move your customization to your child theme, please see https://theme.co/apex/forum/t/setup-how-to-setup-child-themes/57
After your child theme is setup, please review how we recommend making template changes in Customization Best Practices.
Hope this helps!