[Evolution] HTML email always using fixed-width fonts?

[Paul Smith <paul mad-scientist net>]

Hi all; I've just upgraded to Ubuntu GNOME 15.10 which comes with GNOME
3.16 inlcuding Evolution 3.16.5.

Overall everything is fine but there is one bizarre thing that I don't
remember hearing about: even when I send an email in "HTML" mode, the
font is always fixed-width.  This is in "Normal" paragraph mode, without
the "TT" button enabled, writing a brand new email.

One exception is that the Subject line uses a normal proportional-width
font.

I've checked "Mail Preferences" and I have "Use the same font as other
applications" checked, and definitely other applications use a
proportional-width font.  Even when I uncheck that box and specify a
specific font it always chooses a fixed width font.

Here's the weird part: when I use a specific font for Evo and change the
size of the fixed width font, messages in "Plain Text" mode DO change
size, but messages in "HTML" mode do NOT change size.  And, vice versa:
if I change the size of the standard font then messages in "Plain Text"
mode do not change size, but messages in "HTML" do.

But, when I change the "standard font" face, nothing happens: no matter
which one I choose I get the same fixed width font.  Except, if I change
to an oblique font then I get an italicized fixed width font... it's
like the HTML font takes the size, weight, slant, etc. but always uses
the same base (fixed width) font no matter what.

Anyone have any ideas about this?