add transparent logo

Author: mmcky

lectures/_config.yml

@@ -51,6 +51,7 @@ sphinx:
           url: http://www.tomsargent.com/
         - name: John Stachurski
           url: https://johnstachurski.net/ 
+      dark_logo: quantecon-logo-transparent.png
       header_organisation_url: https://quantecon.org
       header_organisation: QuantEcon
       repository_url: https://github.com/QuantEcon/lecture-python-intro