Configuration Registry

<?php

/* Retrieving a single key from a configuration */
$currentTheme = \Drupal::config('system.theme')->get('default');  /* Returns a read-only configuration object */


/* Retrieving an editable object for making configuration changes */
$themes = \Drupal::service('config.factory') -> getEditable('system.theme');
$themes -> set('default', 'gt');
$themes -> set('admin', 'seven');
$themes -> save();

To browse all of your configuration keys, go to the administrative toolbar to Configuration -> Development -> Configuration syncronization -> Export.

The Drupal.org website has a more detailed introduction to Drupal 8 configuration system.

Do note that some configuration objects actually represent entities stored as configuration (instead of being stored in their own database tables).  Examples of this include user roles, content types, and paragraph types (for the Paragraphs module).  You should access these through their appropriate entity handlers, and not try to manipulate them through direct configuration management calls.