Where may I find a reference scheme for GNOME 3 theming (e.g. Adwaita)?

There's only a single line in that particular .css file because the default theme (Adwaita) comes as a binary:

Adwaita is a complex theme, so to keep it maintainable it's written and processed in SASS, the generated CSS is
then transformed into a gresource file during gtk build and used at runtime in a non-legible or editable form.

Since the code has been included in gtk+, you can view the source files HERE. As per their readme:

_colors.scss        - global color definitions. We keep the number of defined colors to a necessary minimum, most colors 
                      are derived form a handful of basics. It covers both the light variant and the dark variant.
_colors-public.scss - SCSS colors exported through gtk to allow for 3rd party apps color mixing.
_drawing.scss       - drawing helper mixings/functions to allow easier definition of widget drawing under specific context.
                      This is why Adwaita isn't 15000 LOC.
_common.scss        - actual definitions of style for each widget. This is where you are likely to add/remove your changes.

The reference .css files are:

gtk-contained.css
gtk-contained-dark.css

Here is an older how-to, some stuff no longer applies (as I said, the source code is in gtk+ now, as gnome-themes-standard was deprecated) but it should get you on the right path:

How to hack GNOME Adwaita GTK theme