Hi Adina,
This error page is independent from the template and it has its own layout (we normally need to customize this 'error.php' file in each template). So in order to have the main navigation like other pages of template, you can edit this file:
ROOT/templates/purity_iii/error.php
so that when there is 404 error, it will redirect to an article page that shows the error as you wish.
Here is the sample:
1/ Create a new article called '404' with the content you want like an image with 404 message or some other text...
2/ Create a new menu item type of Articles >> Single Article >> point the new created article above. In the settings of this menu item, you can set to hide all article's info such as title, ... and show the content only.
3/ Edit the error.php file: After the code defined('_JEXEC') or die; in line 10, you just need to add the following code:
if (($this->error->getCode()) == '404') {
header('Location: ' . Jroute::_("index.php?option=com_content&view=article&id=290", false));
exit;
}
You will notice the link is from the menu item you created in step 2:
