Nice to have
When an application is launched in the Modern theme, there is no admin button in the toolbar.
The reason for this is the changes made to the tl:TabSwitchCommandModelConfiguration by ticket #26373. Previously, it was possible to configure an image for the button at the tl:TabSwitchCommandModelConfiguration. In the adminButtons.xml, IMAGE_BUTTONS_ADMIN_SIMPLE was configured as the icon for administration. After the changes made by ticket #26373, the icon of the arguments of the legacyTabbar.template.xml is used.
The layout of the administration is defined in admin/index.layout.xml. The icon for the button is IMAGE_BUTTONS_ADMINISTRATION22. In the Modern theme the variable IMAGE_BUTTONS_ADMINISTRATION22 does not exist, so the tl:ThemeImage$NoIcon is used, which adds a style="display:none" when rendering the button. This will not display a button for administration.
Start demo in Modern Theme and check if the button is visible for administration.