Fonts cannot be enlarged

Hi,

I am using Balsa 2.6.3 on a recent Ubuntu system.
I cannot enlarge ( View/Zoom ).
I have edited $HOME/.balsa/config and I've set

PrintHeaderFont
PrintBodyFont
PrintFooterFont
MessageFont
SubjectFont

to larger values, but this doesn't seem to have any effect.

On startup I get the following error messages
(balsa:4425): Gtk-WARNING **: 12:08:50.571: Theme parsing error:  
<data>:7:13: Not using units is deprecated. Assuming 'px'.
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Junk at end of value for font-size
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Not using units is deprecated. Assuming 'px'.
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Junk at end of value for font-size
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Not using units is deprecated. Assuming 'px'.
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Junk at end of value for font-size
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Not using units is deprecated. Assuming 'px'.
(balsa:4425): Gtk-WARNING **: 12:08:50.572: Theme parsing error:  
<data>:7:13: Junk at end of value for font-size
(balsa:4425): Gtk-WARNING **: 12:08:50.609: Theme parsing error:  
<data>:7:13: Not using units is deprecated. Assuming 'px'.
(balsa:4425): Gtk-WARNING **: 12:08:50.609: Theme parsing error:  
<data>:7:13: Junk at end of value for font-size
(balsa:4425): GLib-GIO-WARNING **: 12:08:55.476: unable to send  
notifications through org.freedesktop.Notifications:  
GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name  
org.freedesktop.Notifications was not provided by any .service files


Many thanks for some hints,
Helmut