Joomla! uses the templates/system/error.php file to handle several HTTP Status errors, including "403 Forbidden", "404 Not Found", and "500 Internal Server" Errors. You can style the error results, if desired. It is important to understand that the error.php is an independent file from Joomla!. Plugins do not run on the file. You cannot include modules or use  statements.

Overriding the System Error Results

To override the system error results, copy thetemplates/system/error.phpfile into yourtemplates/directory.
If it finds one, Joomla! will use the error.php file from the current template, in place of the system file.
You can format the page, as desired, to match your template.

Overriding the System Styling

If you want to change the styling, copy thetemplates/system/css/error.cssfile into yourtemplates/cssdirectory.

Next, update the your templates/error.php file to reference the new location of the stylesheet by changing this line, accordingly:

"stylesheet"href="/baseurl; ?>/templates/system/css/error.css"type="text/css"/>

Then, simply change the error.css, as desired, for your styling requirements.

Customizing Error Messages

To create the custom error message, dependent upon the specific error code, you'll want to add the following code somewhere in the error.php document to create the custom message.