This makes some things look nicer (e.g. adds gradients, etc).

Now the style is matching closely `.table-bordered` from Bootstrap.
I decided to leave the header row background blank because it was too
close in color to `#F9F9F9` - our new rule for even rows in the table.

I also increased horizontal padding by 5px on both sides, because now
that we have vertical borders it was too cramped.

Additionaly I introduced borders for every cell in the table.

Previously: table cells had no padding, which caused a visual collapse
(cell content was too close to cell neighbors). Table header rows had
dark blue background and black letters.  Tables had no space between
them and following paragraphs.

Changes:
* added 5px padding to cell contents
* changed header row background to gray-ish (I considered switching text
  color to white, but it didn't look good)
* 15px margin on the bottom of the tables (similarly to bottom margin
  used by paragraph style).

As per swcarpentry/lesson-template#140, but only the core CSS
changes.

-   Remove extra space before title of boxes
-   Remove background color of boxes when printing
-   Add "Input", "Output" and "Error" tips for code blocks
    when printing
-   Avoid duplicate lines

and "learning objectives" box.