[fix] sphinx 3.1 CSS issues

See sphinx-doc project, PR 7838 & 7484 with elementary patch to the basic CSS:

- https://github.com/sphinx-doc/sphinx/issues/7838#issuecomment-646009605
- https://github.com/sphinx-doc/sphinx/pull/7484#issuecomment-646058972



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>