A good place to start is creating a plugin to replace adding code to your functions.php file and making it “must use”.
That way your code additions are independent of your theme. It’s basic and useful and won’t be accidentally removed, or disabled.
http://wpninjas.com/how-to-create-a-simple-wordpress-plugin/
https://premium.wpmudev.org/blog/why-you-shouldnt-use-functions-php/
Hope that helps!
PS I’m new, so please excuse me if I’ve done something wrong!