A very simple darker modification of the standard theme for Gnome 3.16.
To change the color palette,
simply play with colors in gnome-shell/gnome-shell-sass/_color.scss
and generate css by running
sass --sourcemap=none --update .
in the gnome-shell folder.
When having User Themes extension installed, you can do this directly in your ~/.themes folder and run rt via Ctrl-F2 to reload the theme immediately.