How to add special text format on GitHub

This is by design as you should not be serving or linking to raw files in this manner.

It could also be seen as abuse of their services as linking to the raw file is using GitHub as a content delivery network (CDN).

Instead, you need to make use of the GitHub Pages feature and link to the file from there. Which, when you build your pages will be along the lines of:

<link rel="stylesheet" href="http://repo-owner.github.com/repo/css/style.css">

In your case, it would be here:

<link rel="stylesheet" href="http://dublintech.github.com/JavaScript_Examples/jsquiz/css/jquiz.css">

And when it is referenced from the correct location, it will be served as text/css as expected/wanted.

When posting to Twitter, Google+ or Facebook, the only logo that will come through is the image attached to the owner or organisation that owns the repository.

As such, to edit what that logo is, you will have to update the linked Gravatar image attached to the email on that owner or organisation account.

You could add an image to the repo itself, but that will often not come through as an option for the thumbnail/image that's shown with the link. Doesn't matter if the image is part of the repository, or hosted elsewhere, it won't show up.