Purpose

To remind myself how to do this again in the future.

How to do it?

To append a Rouge theme to an existing file you can do the following:

gem install rouge
irb

require "rouge"
theme = Rouge::Themes::Github.new.render
File.open("<path-to-file>", "a+") { |f| f.write(theme) }

All themes

https://github.com/rouge-ruby/rouge/tree/master/lib/rouge/themes