Added dark mode screenshot

Author: vignesh-ragupathy-sage

README.md

@@ -44,8 +44,14 @@ docker run -d -p 8000:80 --name plotly4nagios vignesh88/plotly4nagios
 
 ## Screenshot
 
+### Default mode
+
 ![Alt text](img/screenshot.png?raw=true "Title")
 
+### Dark mode
+
+![Alt text](img/screenshot_darkmode.png?raw=true "Title")
+
 ## License
 
 Copyright 2020-2021 © Vignesh Ragupathy. All rights reserved.