Change the colors in the Sphinx documentation

Basically, everything that used to be blue is now green, and everything that
used to be white (text) is now a light gray.  The theme is based off of the
SymPy website, SymPy Live, and SymPy Gamma.

I am not a web designer.  Probably all of this could be done a little bit
better.  Any suggestions are welcome.
Merge pull request #1792 from asmeurer/doc_colors

Change the colors in the Sphinx documentation
Fixed the infinite rescursion.
Added an extra condition that takes an expression with an order term
so that the expression could be added to another order term without
reaching maximum recusion depth.
Merge branch 'master' of https://github.com/sympy/sympy into series-expand