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>

Documentation/Makefile

@@ -20,6 +20,7 @@ SPHINXBUILD   = sphinx-build
 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 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
 	-D version=$(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 @@ dochelp:
 	@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'

Documentation/conf.py

@@ -210,6 +210,7 @@ highlight_language = 'none'
 
 # Default theme
 html_theme = 'sphinx_rtd_theme'
+html_css_files = []
 
 if "DOCS_THEME" in os.environ:
     html_theme = os.environ["DOCS_THEME"]
@@ -229,6 +230,12 @@ if html_theme == 'sphinx_rtd_theme':
     except ImportError:
         html_theme = 'classic'
 
+if "DOCS_CSS" in os.environ:
+    css = os.environ["DOCS_CSS"].split(" ")
+
+    for l in css:
+        html_css_files.append(l)
+
 if major <= 1 and minor < 8:
     html_context = {
         'css_files': [],

Documentation/doc-guide/sphinx.rst

@@ -138,6 +138,9 @@ To pass extra options to Sphinx, you can use the ``SPHINXOPTS`` make
 variable. For example, use ``make SPHINXOPTS=-v htmldocs`` to get more verbose
 output.
 
+It is also possible to pass an extra DOCS_CSS overlay file, in order to customize
+the html layout, by using the ``DOCS_CSS`` make variable.
+
 By default, the build will try to use the Read the Docs sphinx theme:
 
     https://github.com/readthedocs/sphinx_rtd_theme