How to shade the whole area after fill-column with display-fill-column-indicator-mode

[kotatsuyaki]

Hi all,

I am seeking for a way to shade the whole area after fill-column with a
grey background, as seen in this screenshot [1] from the page for "Fill
Column Indicator" [2] on the EmacsWiki.  How do I achieve the style
using display-fill-column-indicator-mode?

I tried setting the :extend face attribute on the fill-column-indicator
face, as that's how I make the background color of the headlines in
org-mode extend all the way to the right until the edge of the window.
However, the background color of the indicator does not extend:

(set-face-attribute 'fill-column-indicator
                    nil
                    :background "LightGray"
                    :extend t)

[1] https://www.emacswiki.org/pics/static/FciShadingStyle.png
[2] https://www.emacswiki.org/emacs/FillColumnIndicator