Assuming that you made changes to the base css file in cpanel instead of the custom css in the admin panel. I would download the simplicity theme, create a folder in cpanel, and upon it there. Unzip it, and copy the base css file into the ow_themes/simplicity folder to overwrite the file. This should take you back to the original settings.
Just a note. Always make your changes through the custom css in the admin panel. This leaves the original unchanged.