You are getting fatal errors but your WordPress is configured so that these errors are not displayed in front page for security reasons. However, all error messages can be found in your hosting log files and that’s where you should start your debugging.
There must be a bug in code that you added inside Child Theme functions.php file. This is definitely not related to CSS.
Entire extras.php file can’t be overwritten entirely and you should work with individual functions. Here is some more information about how this process works.