This addon is a theme. You need to extract it to your /themes folder to install it.
This theme is designed to help users see what can be easily changed in the Vanilla default style, so they can get 'theming' quickly. The custom.css file contains only the code needed to make the changes, and each of the css rules is annotated to explain what it affects. The 'style' looks horrible, as I've made colours garish so they will stand out. I've also added a Quick Links side panel and a couple of items to the menu.
This is a much cleaner and clearer css file than my annotated_css1, and should be a lot easier to follow. The most recently updated (2.03.1) has improved annotation.
It should allow relative newcomers to css to make basic changes to the default theme in just a few minutes.
Can't seem to delete the first screenshot. The second is how the most recent theme looks. The third is how it looks on my website.