docs: allow to pass extra DOCS_CSS themes via make

Specially when the RTD theme is not used, it makes sense to
allow specifying extra CSS files via a make variable.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/03d09bf41ad39aa0abfe2ea3c879b09aa3a0948d.1638870323.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index 11f8b3b9..9f0f53d 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -20,6 +20,7 @@
 SPHINXOPTS    =
 SPHINXDIRS    = .
 DOCS_THEME    =
+DOCS_CSS      =
 _SPHINXDIRS   = $(sort $(patsubst $(srctree)/Documentation/%/index.rst,%,$(wildcard $(srctree)/Documentation/*/index.rst)))
 SPHINX_CONF   = conf.py
 PAPER         =
@@ -85,7 +86,10 @@
 	-D version=$(KERNELVERSION) -D release=$(KERNELRELEASE) \
 	$(ALLSPHINXOPTS) \
 	$(abspath $(srctree)/$(src)/$5) \
-	$(abspath $(BUILDDIR)/$3/$4)
+	$(abspath $(BUILDDIR)/$3/$4) && \
+	if [ "x$(DOCS_CSS)" != "x" ]; then \
+		cp $(DOCS_CSS) $(BUILDDIR)/$3/_static/; \
+	fi
 
 htmldocs:
 	@$(srctree)/scripts/sphinx-pre-install --version-check
@@ -157,4 +161,6 @@
 	@echo
 	@echo '   make DOCS_THEME={sphinx-theme} selects a different Sphinx theme.'
 	@echo
+	@echo '   make DOCS_CSS={a .css file} adds a DOCS_CSS override file for html/epub output.'
+	@echo
 	@echo  '  Default location for the generated documents is Documentation/output'