Make 'org-indent' always monospaced

This is essential for 'org-indent-mode' to align entries correctly.

Thanks to Steve Molitor for reporting the issue 9 over at the GitHub
mirror: <#9>.  Steven
tested a sample file with 'modus-vivendi' and got the desired results.
It reminded me how I handle this face with the 'modus-themes'.

README.org

@@ -1075,7 +1075,8 @@ matters.
 + Contributions to code or the manual :: Philip Kaludercic.
 
 + Ideas and/or user feedback :: Anthony Chavez, Iris Garcia,
-  Jean-Philippe Gagné Guay, Spike-Leung, Summer Emacs, Zoltán Király.
+  Jean-Philippe Gagné Guay, Spike-Leung, Steve Molitor, Summer Emacs,
+  Zoltán Király.
 
 * GNU Free Documentation License
 :PROPERTIES:

ef-themes.el

@@ -1185,7 +1185,7 @@ Helper function for `ef-themes-preview-colors'."
     `(org-headline-done ((,c :inherit org-done)))
     `(org-headline-todo ((,c :inherit org-todo)))
     `(org-hide ((,c :foreground ,bg-main)))
-    `(org-indent ((,c :inherit org-hide)))
+    `(org-indent ((,c :inherit (fixed-pitch org-hide))))
     `(org-imminent-deadline ((,c :inherit bold :foreground ,err)))
     `(org-latex-and-related ((,c :foreground ,type)))
     `(org-level-1 ((,c :inherit ef-themes-heading-1)))