By now there are quite a few packages that give the mode-line a more
"modern" look, including powerline, spaceline, doom-modeline, mood-line
and my moody.

  https://github.com/milkypostman/powerline
  https://github.com/TheBB/spaceline
  https://github.com/seagle0128/doom-modeline
  https://gitlab.com/jessieh/mood-line
  https://github.com/tarsius/moody

Something that many, if not all, of these packages have in common is
that they use images to give the mode-line a more "graphical" look.

The purely textual parts of the mode-line change their color based on
whether the respective window is the selected window by using one of
the faces mode-line or mode-line-inactive.

The colors of certain images also have to change based on whether the
window is selected or not.  Please look at the attached screenshot to
see why that is so.