Ubuntu 18.04's package repos include the Numix Project's gtk theme and icon theme, but these have an undocumented dependency on gnome-icon-theme, which is not installed by default.
So to make Numix work you need to first:
sudo apt install gnome-icon-theme numix-gtk-theme numix-icon-theme
If not already installed, also add (Gnome) Tweak tool:
sudo apt install gnome-tweak-tool
Then open up Tweaks , go to the Appearance tab, and change:
Themes... Applications to "Numix"
Themes... Icons to "Numix"
That's it!