Deleted the dark colored themes. Override mechanism for themes changed.

 * Deleted Amelia, Cyborg, Slate, Spacelab, Spruce and Superhero
 * main.min.css from the default theme is now always included. If a main.css file is included with the theme, then it will be imported after the default main.min.css so you can override the main.min.css rules from main.css or leave them if you'd like.
 * Added additional top-padding for the themes that had a fat navbar.
 * Made the additional themes responsive by including bootstrap-responsive.css.