I've been reading the information about renderers here: http://docs.moodle.org/dev/Overriding_a_renderer (and other pages). I think I'll be able to follow the instructions given to change themes but there's a question that's unanswered by the tutorials.
Is it possible to link the render override to data in the database e.g. if a custom user field has a high contrast check box checked then apply a renderer override? Or have I got this all wrong?