How to change the default "site name :: page title" formatting in 5.7

In your application\config folder, open concrete.php. If you don't have a concrete.php file, you will need to create one. The concrete.php file will only contain PHP, so only an opening PHP tag is required. Paste the following code into concrete.php.

return array(
    'seo' => array(
        'title_format' => '%1$s :: %2$s',
        'title_segment_separator' => ' :: ',
    )
);

-- this is a copy of the default configuration settings for formatting the site name, page separator, and page title

If concrete.php exists and already has arrays of configuration settings. Add a comma after the last first level subarray (not the returned array) and then paste the formatting configuration settings after the comma.

Example: existing array

return array(
    'marketplace' => array(
        'enabled' => false
    ),
    'external' => array(
        'news_overlay' => false,
        'news' => false,
    )
);

-- 'external' is the last first level subarray

Example: changing the separator to a hyphen

return array(
    'seo' => array(
        'title_format' => '%1$s - %2$s',
        'title_segment_separator' => ' - ',
    )
);

Example: adding to the existing array

return array(
    'marketplace' => array(
        'enabled' => false
    ),
    'external' => array(
        'news_overlay' => false,
        'news' => false,
    ),
    'seo' => array(
        'title_format' => '%1$s - %2$s',
        'title_segment_separator' => ' - ',
    )
);

The first array key 'title_format' has a string value that has a placeholder for the site name, separator, and a placeholder for the page name.
The second array key 'title_segment_separator' has a string value that is the separator.
The separator in title_format and title_segment_separator must match.