LUbuntu 20.04 LTS “Focal Fossa” ships with a built-in dark theme. You can activate the dark version of Ubuntu’s standard Yaru theme and get a dark GNOME desktop in just a few clicks. Here’s how.
First, click the system menu button on the panel at the top-right corner of Ubuntu’s GNOME desktop and select “Settings.”
You can also click the “Show 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.
By default, Ubuntu uses the “Standard” window color theme with dark toolbars and light content panes. To activate Ubuntu’s 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.
Note that this won’t change Ubuntu’s panel interface to dark mode. For example, when you click the menu button at the top-right corner of your screen, you’ll see that it has a light background even when you have dark mode enabled.
To change the panel theme, you’ll 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 switching the “Shell” theme to “Yaru-dark.”