LUbuntu 20.04 LTS "Focal Fossa" comes with a built-in dark theme. You can activate the dark version of Ubuntu's default Yaru theme and get dark GNOME desktop in just a few clicks. Here's how.
First click the system menu button on the panel in the top right corner of Ubuntu's GNOME desktop and select "Settings".
You can also click the "View Applications" button at the bottom left. corner of your screen, search for the "Settings" application, and launch it from there.
Click the "Appearance" category in the Settings application.
Ubuntu defaults to the "Standard" color theme with dark toolbars and light panels. To activate Ubuntu dark mode, click "Dark" instead.
To use a light mode without the dark toolbars, click "Light" instead.
 Your change will take effect immediately. You can now close the window.
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Note that this does not change Ubuntu's panel interface in dark mode. For example, if you click on the menu button in the top right corner of your screen, you will see that it has a light background even if you have turned on dark mode.
To change the panel theme, you need to install a GNOME Shell extension. OMG! Ubuntu! has a guide that will walk you through the process of installing the gnome-shell-extensions package, enabling the "User Themes" extension and changing the "Shell" theme to "Yaru-dark".