All of my custom CSS codes are completely gone after changing theme

While I was changing the background image setting and check out different theme.  All my custom Css statements are completely gone. Please help.  

It's when changing a theme in the form builder. The form builder resets the CSS to avoid any issues in the styles. If you want to change some styles of the theme, you need to do it after it's being applied. 

If you want to restore the Custom CSS codes, you need to revert your form to the earlier version:

https://www.jotform.com/help/294-How-to-view-form-revision-history