CSS is theme specific. If you have custom CSS and later change themes, you need to delete whatever CSS you have in the Editor, otherwise, you’ll get undesired results.
To ‘reset’ a theme, you need to delete the CSS in the Editor, make sure the “add to existing CSS” radio button is checked, then save. Things should go back to normal.
When you alter a theme’s stylesheet, you only need to put in the Editor the modification/additions to the CSS, not the entire thing. Read this for more info:
We have only 2 Volunteers who help with CSS editing occasionally so please follow the instructions above and be patient waiting for them to log-in, find this thread, and help you.