Add white background to logo in README for dark mode. Closes #1268.

README.md

@@ -1,6 +1,6 @@
 <a href="https://zerodha.tech"><img src="https://zerodha.tech/static/images/github-badge.svg" align="right" /></a>
 
-[![listmonk-logo](https://user-images.githubusercontent.com/547147/134940003-1de03d83-8c7b-459b-8056-baa8d5f3b448.png)](https://listmonk.app)
+[![listmonk-logo](https://user-images.githubusercontent.com/547147/231084896-835dba66-2dfe-497c-ba0f-787564c0819e.png)](https://listmonk.app)
 
 listmonk is a standalone, self-hosted, newsletter and mailing list manager. It is fast, feature-rich, and packed into a single binary. It uses a PostgreSQL (⩾ v9.4) database as its data store.