Remove the explicit setting of a pygments theme (#17571)

This allows the underlying theme being used to provide the pygments
theme instead which looks better (subjectively).

Author: pradyunsg

docs/source/conf.py

@@ -92,7 +92,7 @@
 # show_authors = False
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = "sphinx"
+# pygments_style = "sphinx"
 
 # A list of ignored prefixes for module index sorting.
 # modindex_common_prefix = []