Adding an explicit theme corresponding to the old 'default' theme of sphinx before 1.3.
See upstream issue #9273 ( https://github.com/sympy/sympy/issues/9273 ).

--- doc/src/conf.py
+++ doc/src/conf.py
@@ -95,6 +95,7 @@
 
 html_logo = '_static/sympylogo.png'
 html_favicon = '../_build/logo/sympy-notailtext-favicon.ico'
+html_theme = 'classic'
 # See http://sphinx-doc.org/theming.html#builtin-themes.
 html_theme_options = {
     'collapsiblesidebar': True,