The theme you use when editing your site is not necessarily the same as the one users see when they are browsing your site. Drupal 8 includes a specially designed admin theme.

Go back to the Appearance page:

At the bottom of the appearance page is a panel allowing you to specify the Administration theme:

The checkbox underneath the theme selection allows you to specify which themes (the frontend theme or the admin theme) are used when the content is being edited.

As a rule, using the administration theme will give you a better editing experience.