How can I create themes for Gnome Shell?

This is kind of an old question, Anyway there is a new GTK GUI inspection tool added with GTK v3.14.

GtkInspector is the built-in interactive debugging support in GTK+. It was added in GTK+ 3.14, based on a copy of the well-estabished gtkparasite.

To enable the debugger, either start your app with GTK_DEBUG=interactive in your environment, or run gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true and launch the debugger with the ControlShiftI or ControlShiftD keyboard shortcuts.

Source: Gnome Projects: GtkInspector

It is more convenient to know the runtime GUI shell directly than following code, as many applications out there using chunks to construct GUI dynamically.

I didn't need to use the shortcut running for example GTK_DEBUG=interactive gedit brings up the GTK Inspector.

while i also have failed so far to find some type of cheat-sheet for gnome 3.4 themes the posts on the blog of fp.murphy where Lucas Romero already has linked to seem to contain plenty of very useful information regarding gnome 3.x and its customization.

some basics worth reading prior to the post provided by Lucas can be found in an ealier post there giving some simple examples and general overview.

if more specifically geared to gnome shell extensions i found the series of up to now 4 posts all written in september 2012 on mathematicalcoffee very helpful. since gnome 3.4 has brought many api-changes its important to look into recent information regarding gnome customization.