font of title from an external variable

[antonfonta]

Hello

I would like to set, in a file called "example.ly", title, tilte font size and title font using variables an external file "example-global.ly"

example.ly is like follows:
\version "2.18.1"
\include "example_global.ly"
\header {
  title = \markup {
         \override #'(font-name . font-titolo)
         "Esempio"
 }
}
\paper {
  #(define fonts
    (make-pango-font-tree  "Agency FB"
                          "Agency FB"
                          "Agency FB"
                         (/ staff-height pt 25)))
}

\score {
c'1
}

File example_global.ly is as follows:

titolo = "Example1"
font-titolo= "Castellar"

Unluckely this doesn't work, I get title in "Agency FB" font.

Instead if I change

\override #'(font-name . font-titolo)

by

\override #'(font-name . "Castellar")

I get title in Castellar font, as desidered.

Does anybody knows the solution, please?

Thanks for your help.