The system tray menu, the calendar in the top bar, notification pop-ups, and desktop context menus are not dark, and they are always on when you switch between Light, Dark, and Standard modes.
Enable global dark theme in Ubuntu 20.04:
1.) First install the user theme Gnome Shell extension.
When I use Ubuntu 18.04, it can only install extensions through Ubuntu Software. However, when I search for user themes in the new Ubuntu software (snap storage), Ubuntu 20.04 shows "Application not found".
As a workaround, open the Firefox web browser and go to https://extensions.gnome.org/ :
Click the link text " click here to install browser extension " and follow the pop-up window to install the browser extension:
Then open the terminal via Ctrl + Alt + T and run the command:
Finally, go to https://extensions.gnome.org/extension/19/user-themes/ .
Open On, and then choose to install the Gnome Shell extension:
2.) If so, please install Gnome Tweaks.
3.) Finally, start Gnome Tweaks and navigate to the appearance in the left pane, select Yaru-dark as the Shell theme and complete.
Of course, if not, please select "Dark" in "System Settings"-> "Appearance" to get the dark application.