Comment 2 for bug 1649319

The problem is from the skin "light" in the TinyMCE theme. He miss the required font and :before css style to call the icons.
A workaround is to replace the call of the "light" skin to the "lightgray" default skin in the htdocs/lib/web.php file.

I don't know exactly where is came from the "light skin" but if its an adapted version from the link below, it has the same problem :
https://pixabay.com/fr/blog/posts/a-modern-custom-theme-for-tinymce-4-40/

Maybe a solution is to begin from the "lightgray" skin and to create a new one. Ex using this:
http://skin.tinymce.com/