We don't know your custom css code, So we can't undo that.
theme.css file will be regenerated again when you save the theme option panel.
Means that theme.css file is often changed by scss compiler. So please don't add your custom css code in there in the future.
Sorry for our work. We can't undo your custom css code. Because compiled theme.css file can't be recovered.
Did you add many custom css code in there?
Best Regards, C-Themes