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.

`<tbody/>` is automatically inserted, so we can guarantee that this affects all ths and tds in the last row of the table *body* and the head is left untouched

The alignment information were already in the generated html.
The css was the only thing missing, so I added it.