I want to make some changes in my shella theme and i dublicated it in order not to loose sth in case i make sth wrong. My question is where do i make the trial changes? On the dublicate or in the normal theme? And in case i made sth wrong and i dont want them the theme will be empty and i have to start from the beggining or everything will be vissible until the point i've made the duplicate?
Take a backup of your theme. Make changes on the backup, keep the original as is. If you like the changes you made on the backup, then publish it.
To be on the safer side, you can take 2 backups.
You can develop on any theme, but you should back up a theme to make back up if you doing wrong or you just want to backup the old version.
You can refer to this link: https://help.shopify.com/en/manual/online-store/themes/managing-themes/duplicating-themes
Hope this helps!