CSS is http://pastebin.com/n3RjP8g9
JS that just makes the theme dark is http://pastebin.com/Z5eEnVUA (Note: I've not tested this, so it may not work, you may have the issue where it will keep adding the same class after every time you refresh.
In case this is the case, I've attached my code below that I use, remove bits until it works how you want it to.
EDIT: The CSS goes in Custom CSS, the Javascript goes in Custom Header under Appearance Custom HTML & CSS in your ACP.
