theme: simplify the main layout

* Simplifies the header and menu layout for all screen width.
* Uses only one background color (Darkest Polar).
* Change the main text color to Snow Light Grey.
* Change the header and footer text color to Snow Lightest Grey.
* Add a color for active and focused links (Frost Lighter Green).
* Simplifies the figure layout and colors.
* Adds a font-family variable and set it to the body. It uses the system
  default font.
* Tries to set all font properties in the _fonts.scss file.
* Removes the links decoration on the header and footer.
* Harmonizes the footer accordingly to the header.
* Uses the hugo regular method to generate the footer menu.
* Stops centering the content on large screens.
* Adds the content max width and content left margin to the variables.
* Sets the font weight to bold for hovered links.
* Suppress the main HTML tag of the index template, and set the
  correct classes on the main HTML tag of the baseof template.
* Uses a second level header for the title of the page, as the first
  level is used for the website title.
* Improves the layout of definition lists on the home page, with a
  better (and simpler) rendering on small screens.
* Prevents to use the post information partial on static page. This is
  done through a test on the layout property `static` of the front
  header.
* Reorders the sections of the home page and adds anchors to the section
  headers.
* Closes #8, #11.

Co-Authored-by: iGor milhit <igor@milhit.ch>