I have site running Moodle 2.9.3+ and i was trying to customize the renderer of one existing plugin, so i found this:
How to override a renderer such that the functi
You need to do two things to get a theme renderer to override a core renderer:
The theme_overridden_renderer_factory works by extending the process of instantiating a renderer to look for a class that matches the name 'theme_NAMEOFTHEME_NAMEOFRENDERER' - as long as that class exists, then it should be used (otherwise the original renderer is used).
See https://docs.moodle.org/dev/Overriding_a_renderer for more details.