Update colors to indigo/green, bit better than purple. (#69)

Only named colors work, and which ones they are can be seen in
https://bashtage.github.io/sphinx-material/customization.html#configuration-options

Author: rgommers

spec/conf.py

@@ -86,8 +86,8 @@
 
     # Set the color and the accent color (see
     # https://material.io/design/color/the-color-system.html)
-    'color_primary': 'deep-purple',
-    'color_accent': 'cyan',
+    'color_primary': 'indigo',
+    'color_accent': 'green',
 
     # Set the repo location to get a badge with stats
     #'repo_url': 'https://github.com/project/project/',