Including style files

[Simon]

Hi,

I'm a beginner at lilypond and I am getting confused trying to do something
which seems to be a very natural and frequently occuring task to me, but for
which I cannot find proper documentation and/or examples on the web and in the
manual.

I want to create a collection of many short tunes, each one living in a separate
lilypond file. For these tunes I want to have a uniform layout which I want to
be able to change quickly for all files. So the natural idea is to move the
necessary definitions to a separate file, say "tune.ly" and read them with 

\include "tune.ly"

at the beginning of each tune. The only thing I managed to do so far is setting
the global font size because I can do that just by adding a line of pure scheme
as described in the manual. However, now I want to change the spacing between
the staves, e.g., using

\set Staff.minimumVerticalExtent = #'(-3 . 3)

Of course it doesn't make sense to put that line into the style file. Reading
Section 4 "Changing Defaults" of the lilypond manual I don't see a single
solution that does not require to add at least some lines nested inside the main
file, e.g. inside the \paper block, which I don't want. I must be missing
something very obvious. Can anyone help or maybe provide a "style file" that was
used for a similar project?

Thanks in advance,
Simon