LilyPond has well-known ways to change the font within markup and for the whole document. It can be harder to find the ways to change the font for all of just a specific class of text, such as lyrics; but it can be done. Try:
\override LyricText.font-name = #"Comic Sans MS Italic"
It is possible to get round this by defining your own font family in addition to those provided within LilyPond, like this:
\paper {
#(define fonts
(let ((n (set-global-fonts))
(factor (/ staff-height pt 20)))
(add-pango-fonts n 'handwriting "Comic Sans MS" (* 1.1 factor))
n))
}
#(define-markup-command (handwriting layout props arg) (markup?)
(interpret-markup layout
(prepend-alist-chain 'font-family 'handwriting props) arg))
You can then set your newly-defined font-family to be used, after which the usual facilities are still available, as in:
\lyricmode {
\override LyricText.font-family = #'handwriting
Blah blah
\override LyricText.font-shape = #'italic
blah
\once \override LyricText.font-shape = #'normal
blah
...
}
The override command can be put at the start of a particular set of lyrics if you want only that verse or whatever to use the new font, or in a \layout block to change the font for all lyrics.
Paul