We don't use the listed content managers. Can I still use CiviCRM for contacts?
What you can do is set up a new subdomain https://secure.yourdomainname.org -
Then run a very minimal Drupal (or Wordpress) + CiviCRM on that secure.yourdomainname.org - this will provide you with all the benefits CiviCRM can offer and help you to consolidate things for your organization - without the need for your staff to learn how to use Drupal for content editing etc.

If in future you want more integration between your website and CiviCRM (esp in Drupal there are lots of integration options [e.g. views and my personal favourite: webform_civcrm module] that extend what CiviCRM can do for you) - then you can consider moving your SilverStripe website to Drupal!