Upgrading Templates in the system

Hello,

I renamed my custom-sample.css to custom.css and added my own css styles to it for the theme Blog.MU which is the default Theme that customers come to when signing up for my program.

I also copied all my css styles to the child-style.css file. When I upgraded my theme today with updates you guys sent out, the css files I had made custom.css was removed and the child-style.css was overwritten.

I was able to just reupload them but I was hoping to prevent changes to the site upon updating the theme. How would I prevent this in the future?

Thanks for your help