Hi,
What I wrote earlier was wrong.
What you need to do is add a pix_plugins folder to the Adaptable theme.
Here is a link to the Afterburner theme that uses some custom icons like this example:
https://github.com/lazydaisy/moodle-theme_afterburner/blob/master/pix_plugins/mod/lesson
And here is the pix_plugins folder itself that I added to that theme when I first created Afterburner for Moodle.2.1
https://github.com/lazydaisy/moodle-theme_afterburner/blob/master/pix_plugins
hope this helps?
Mary