Administration forms tend to be wide and long, sometimes not fitting too well within the confines of the content area defined for a given theme. To address this problem, Drupal 8 lets you specify a theme that should be used for administrative functions. You can try your new theme to see if it works for administration screens, or you can pick a different theme to use whenever a site administrator is performing site administration tasks. Typically, a simple clean theme that is at least 960px wide works best as the admin theme. To change the administration theme, simply click the Appearance menu item at the top of the page and scroll to the bottom. You’ll see a section titled Administration Theme. From the list of themes in the drop-down list, select a theme that you know will work with administration forms. By default Drupal 8 enables Seven as the administration theme, because it accommodates administration screens. You may also change the administration theme to any other theme listed in the drop-down list. If you change the value, make sure you click the “Save configuration” button. Most drupal developers are working with this theme.
Drupal 8 provides a set of configuration options that, when changed, updates certain aspects of what is displayed within your theme. Depending on whether the theme author adhered to Drupal standards, you can use this form to determine which elements are displayed on the page (Logo, Site Name, Site Slogan, and so on), whether the theme should use its default logo, and whether the shortcut icon should be used (the shortcut icon is also known as the favicon; it’s the little logo that appears to the left of your browsers address bar). To get to this screen, simply click the Appearance menu item in the top menu and click the Settings tab at the top of the Appearance page.
Try changing some of the display options such as the logo. Uncheck the “Use the default logo supplied by theme” check box and use the Browse button that appears after unchecking the box to upload a new logo for your site. Click the “Save configuration” button at the bottom of the Settings page and return to your site’s homepage to see the change in appearance.