Convert to css went wrong

I’m working on a project, it is required to use of user-defined CSS, thinking of me that I need to theme editor.
Therefore I used a theme editor and by mistake (note: I didn’t know that this going to happen) I clicked on convert to CSS and everything went wrong the original style of the website is gone.

How can I fix this and revert everything, please?

This feature is quite buggy. If your hosting offers backups, it is recommended to restore the store from backup