Despite this being a "dpkg-source 3.0 (native) format" package, it uses quilt
to patch the gfxboot theme code imported from openSuSE. The siduction patch
series is located under debian/patches/, to configure quilt to use this
location instead of its default (patches/) location, you can supply:

QUILT_PATCHES=debian/patches

in your environment.

Basic patch operations contain:

$ QUILT_PATCHES=debian/patches quilt push [-a]
  to apply the next [or all pending] patches

$ QUILT_PATCHES=debian/patches quilt pop [-a]
  to unapply the last [or all applied] patches

$ QUILT_PATCHES=debian/patches quilt new <patch>
  to start a new patch

$ QUILT_PATCHES=debian/patches quilt refresh
  to update the last applied patch of the current queue

A good howto for Debian specific quilt usage can be found under
/usr/share/doc/quilt/README.source (as part of the quilt package)
or in quilt's manpages.
