Hello Bionicnelly,
Thanks for the question! Changing the error pages so that they display a custom error page is what I believe you are asking. The error.php is the file that handles the HTTP status errors - it doesn't actually change how they look. I found documentation on how make these changes directly with Joomla here: Custom Error Pages. Also, we have an article on Joomla-generated error pages if needed.
I hope this gives you the information you need to modify your error pages. If you still require further assistance, please let us know.
Regards,
Arnel C.