Customize current theme, or create a new one?

I wonder if it is a good idea to change current theme files in domain/content/themes/theme_name/ or we must create a copy from the current theme as our customized theme and then change that one?
I wonder if I change the current theme’s files, if there is an update for the theme in the future, my changes could be overwritten or not?.
Or is there anything like an update for themes that overwrite customized changes?
I’m from PHP/Laravel and we could not change code from other packages that are in vendor folder, because future updates for the package will overwrite our changes. I wonder if the ghost has something like that.
Thank you for your response.

Upadte Depend’s On Your Server’s Installed Ghost Version. If Ghost Authority Update There Version, You Would Update Your Server Version Manually. If You Don’t Update Your Server, Your Server Ghost Version Remain Old Version.It’s Not Update Automatically. If You Change Anything In Your Theme and server ghost version supported That It Would Automatically Update. If Not Update Automatically , You Would Restart Your Server.