Overriding header fonts in a template

[Jeff Harris]

I am trying to modify header fonts for the requirement of a publisher.

I would rather not use text markup commands in the header, as it's not
simple to change multiple works at the same time.

I would much rather put the style changes in an external file, so they can
all be changed at the same time if requirements change. I'm using an
external file for paper size and margin, and staff size, so I would prefer
to put overrides in the same place.

Is it possible to put header font-name and font-size overrides in an
external file and how? Maybe some sort of lilypond css?

I tried this but the re-defining of title and other header data didn't work
for me.

\version "2.18.0"
%\header { %% remove header block to turn title into a variable
   title = "Song Title"
   ...
%}
... (\PartMusic and \PartWords definitions }
\header {
   title = \markup { \override #'(font-name . 'Nimbus') 
            \title }
}
\score { \new ChoirStaff << ... >> }