Most professional developers will either work offline on a local server using an app such as MAMP, XAMPP or similar, or they will have their own live ‘staging’ server.
This isn’t really a theme issue, you would have the exact same issue no matter what theme you happen to be using. For issues that are not related to a theme you can ask for help in the appropriate WordPress support forum. https://wordpress.org/support/
References:
https://codex.wordpress.org/Installing_WordPress