Add CSS for documentation.
Add CSS for documentation.

Ref: http://git.savannah.gnu.org/cgit/datamash.git/tree/doc/datamash-texinfo.css

Mentioned in: http://www.gnu.org/software/texinfo/

ProofGeneral/proofgeneral.github.io,ProofGeneral/proofgeneral.github.io,ProofGeneral/proofgeneral.github.io