-SOURCE_PNG_IMAGES=$(shell ls $(top-src-dir)/Documentation/user/*.png)
-OUT_PNG_IMAGES=$(SOURCE_PNG_IMAGES:$(top-src-dir)/Documentation/user/%.png=$(outdir)/%.png) $(outdir)/context-example.png
+CSS_SOURCE_FILES = $(shell ls $(top-src-dir)/Documentation/lilypond*.css)
+SOURCE_PNG_IMAGES = $(shell ls $(top-src-dir)/Documentation/user/*.png)
+OUT_PNG_IMAGES = \
+ $(SOURCE_PNG_IMAGES:$(top-src-dir)/Documentation/user/%.png=$(outdir)/%.png) \
+ $(outdir)/context-example.png