When you are attaching a custom css file to a project. Leave it always in the same place. For example when am working with custom css for our websites. I have it in scripts folder. Custom css file is edited with Sublime and every change is saved. Everytime when the project is exported it will take the custom css file from there and include it in project css folder.
Include the custom css file in project setting so it will appear on all your pages. Or if you just need on specific page add it in page preferences.
So, but when you write your custom css class you need to add it to whatever element you want or need in blocs. Just create a new class with the same name and add it to a element. By export it will have it in source code as a class name but the all class styling it will take from that external css file.
So not sure why do you need to reattach it everytime.
All classed made internali in blocs goes to style.css file as a custom entrie.
No need to change that file