It turns out (he answered himself) that this functionality can be adjusted with CompizConfig Settings Manager.
(To install CompizConfig, enter sudo aptitude install compizconfig-settings-manager
from a terminal. It's also, as of this writing, available through the Ubuntu Software Centre.)
To adjust the transparency of non-active windows (note these instructions have only been tested on version 14.04):
- Start CompizConfig.
- Go to the Effects menu (either click on Effects in the left pane or click All and scroll to the Effects section)
- Click TrailFocus. Its description is "Adjust the opacity, saturation and brightness of windows based on when they last had focus."
Here you can choose either
The Behaviour tab, to configure
- which window types (e.g. toolbar, utility, dialog, normal) that TrailFocus will affect
- the number of windows that TrailFocus will track (beyond which number it will consider a window "completly (sic) unfocused")
- which window in the switcher stack to start fading
The Appearance tab, to configure the opacity, brightness and saturation levels of both focused and unfocused windows