Why is my page all black with dark grey text?

You probably have Bartik set as your active theme. This has a dark background to the page. To change this, set Site Studio minimal as your default theme.

You've not set the Site Studio minimal theme as the default, so your website is still showing styles from Bartik (Drupal default theme).

Navigate to 'Appearance' and click 'Set as default' next to Site Studio minimal theme.