Open Tools -> Command Palette.... Search for Package Control: Install Package and click enter. Wait for the available packages to show up and then search for GitHub Color theme. Click enter and the theme should be installed.

Locate your Sublime Text 2 Packages directory by using the menu item Preferences -> Browse Packages....

While inside the Packages directory, clone the theme repository using the command below:

git clone https://github.com/AlexanderEkdahl/github-sublime-theme.git

• Download the files using the GitHub .zip download option
• Unzip the files
• Copy the folder to your Sublime Text 2 Packages directory

Preferences -> Color Scheme -> GitHub.

If you want even more GitHub likeness you can add the following to your user settings

{
  "font_size": 12.2,
  "font_face": "Courier",
  "font_options": ["subpixel_antialias"],
  "line_padding_bottom": 1,
  "line_padding_top": 1
}

All contributions are welcome. Ruby, Haskell and HTML highlighting are almost identical to the GitHub highlighting.