Fixed broken URL

* Print background in pdf

Pull request to integrate issue https://github.com/hakimel/reveal.js/issues/1686. Adding the workaround to the theme template to make it work for every theme.

* apply suggestion of @hakimel

Closes #1951.

"proceding" → "proceeding"

This reverts commit b7e0d9b1.

Defines the highlight colour when selecting slide content using Mozilla
Firefox to match other browsers, instead of being the default colour.

This is similar to what is done in the "white" theme.

The `grunt css-themes` command now loops through all Sass files. There is no need to add the newly created file to the task list.