Can't Use Theme Editor

I was trying to insert background music for a client who is using CS-Cart. I used the Theme Editor - Custom CSS without backing-up the theme. I put in the custom CSS code and I pressed publish by accident.

Now I want to go back into Theme Editor to change the Custom CSS code, but when I click on Theme Editor, it opens the new page but the side panel where the editing is done doesn't show.

Is there anyway to reverse the changes I made by perhaps reverting back to an earlier version?

I have checked the logs for the website and this is what I have:

[b]Logs were removed by moderator because they contained the address of the admin panel, the license key, and sensitive information such as emails. The logs only included requests to [i]dispatch=product_updates.get_available[/i], so they probably wouldn't help with troubleshooting the issue.[/b]

Please help me.