Ubuntu 18.04 missing 'Global Dark Theme' Option in GNOME Tweak Tool

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.