Issue #2113911 by LewisNyman, longwave, nod_, rteijeiro: Modal style update.

core/core.libraries.yml

core/misc/dialog/dialog.ajax.js

core/misc/dialog/dialog.jquery-ui.js

core/misc/dialog/dialog.js

core/misc/dialog/dialog.position.js

core/modules/ckeditor/js/ckeditor.admin.js

core/modules/edit/js/util.js

core/modules/edit/js/views/AppView.js

core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php

core/themes/seven/dialog.theme.css

core/themes/seven/jquery.ui.theme.css

core/themes/seven/seven.info.yml