Relevant Content Module Custom/Overwirete Theme The module Relevant Content has a function called

theme_relevant_content_block

Which renders the nodes for the module. The comments say the following

"This is provided (the function) so that an item list is the default, however a themer can
easily override this to make a teaser list or table."

I'm not really sure how to use a 'themer' to overwrite this function. 

Any pointers on how to do it is appreciated.
You are the themer. In your theme, copy that function, but call it YOURTHEME_relevant_content_block. Now change the function to output whatever you want it to.