My GNOME Ubuntu 16.04 had this option under the tweaks tool:
Tweaks > Appearance > Global Dark Theme > On/Off
My same setup on 18.04 is simply missing this option entirely. How do I get the on/off button back? There's not even this Global Dark Theme line item under Appearances at all.
You probably look for the Adwaita-dark theme
gsettings set org.gnome.desktop.interface gtk-theme 'Adwaita-dark'
you will also find a Firefox Extension to make it dark too
https://addons.mozilla.org/en-US/firefox/addon/adwaita-dark/
If you prefer a gui based solution, install gnome-tweaks:
sudo apt-get install gnome-tweaks
and invoke it with
gnome-tweaks&
or your system menu.
Under the menu appearance you can choose in Themes - Applications different themes, e.g. Adwaita-dark.
Other themes can be found on the gnome-look web page. First create a theme directory mkdir ~/.themesand download your theme into this directory. Then open again tweak tools and choose under the appearance the theme you like most. There are really cool themes, like macos, win10, or even other themes there.
In 16.04, there used to be a dedicated gnome-tweak-tool option for "global dark theme" or something like that. This option is missing on newer versions. However, I figured out how to set this old feature manually as my old Unity GTK settings (from 16.04) are still in place for my main user.
The GTK configuration files are located at ~/.config/gtk-3.0/settings.ini and ~/.config/gtk-4.0/settings.ini.
Make sure the files contains the following:
[Settings]
gtk-application-prefer-dark-theme=1
I believe the default is zero. Log out and log back in to apply the changes.
