docs: allow selecting a Sphinx theme

Instead of having RTD as an almost mandatory theme, allow the
user to select other themes via DOCS_THEME environment var.

There's a catch, though: as the current theme override logic is
dependent of the RTD theme, we need to move the code which
adds the CSS overrides to be inside the RTD theme logic.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/bd20adabfd428fd3cd0e69c2cf146aa354932936.1638870323.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/sphinx.rst

@@ -138,6 +138,14 @@ 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.
 
+By default, the build will try to use the Read the Docs sphinx theme:
+
+    https://github.com/readthedocs/sphinx_rtd_theme
+
+If the theme is not available, it will fall-back to the classic one.
+
+The Sphinx theme can be overriden by using the ``DOCS_THEME`` make variable.
+
 To remove the generated documentation, run ``make cleandocs``.
 
 Writing Documentation