KMail HTML viewer: font size

Hi,

is there a way to change the font size in KMail's HTML viewer? I have a 
MacBookPro 12,1 13" (Early 2015) with retina display (HiDPI), and the rendered 
HTML messages appear way too small to be read. 

I've looked everywhere in KMail settings but didn't find a solution. While it 
easy to change the font size of the message body (Settings -> Configure KMail -
> Appearance -> Fonts -> Message Body), this affects only plain text messages. 

After manually setting my display's size in xorg, almost everything else in 
KDE looks good. This issue with KMail is the only last serious pitfall that 
limits usability, and I'd be glad if someone could suggest me a solution.

This is the tweak I did in xorg config to get the correct resolution (227x227 
dpi):

$ cat /etc/X11/xorg.conf.d/90-monitor.conf
Section "Monitor"
    Identifier             "<default monitor>"
    DisplaySize            286 179    # In millimeters
EndSection

Lastly, but this is a minor drawback, is it possible to enlarge the font used 
for the subject line? By default it is smaller than the font used for the 
From, To, and Date fields, and it appears weird.

Cheers