Messed up theme CSS, How can I fix it?

First, you should never edit the theme files directly. If the theme gets updated because of feature enhancements, bug fixes, or security patches, or if the theme has to be updated because of a change to the WordPress core, then your changes will be lost. The recommended way to modify a theme is to create a child theme.

Second, it’s very rare for CSS to be contained in a .php file, it’s almost always in a .css file. Are you sure it’s CSS and not PHP that you’ve messed up?

To recover any part of the file, just go to the Happenstance subversion repository and download the archive.php file that corresponds to the version of Happenstance that you’ve installed.