chore: icon legibility in dark-mode [skip ci]

Author: alexmercerind

README.md

@@ -7,7 +7,11 @@
   <strong>Sponsored with 💖 by</strong>
   <br>
   <a href="https://getstream.io/chat/sdk/flutter/?utm_source=alexmercerind_dart&utm_medium=Github_Repo_Content_Ad&utm_content=Developer&utm_campaign=alexmercerind_December2022_FlutterSDK_klmh22" target="_blank">
-    <img src="https://user-images.githubusercontent.com/28951144/204833199-41d8e5f5-5e53-4171-8dc0-902be4e1ed61.png" width="350" height="auto" alt="Stream Chat">
+    <picture>
+      <source media="(prefers-color-scheme: dark)" srcset="https://user-images.githubusercontent.com/28951144/204903234-4a64b63c-2fc2-4eef-be44-d287d27021e5.svg">
+      <source media="(prefers-color-scheme: light)" srcset="https://user-images.githubusercontent.com/28951144/204903022-bbaa49ca-74c2-4a8f-a05d-af8314bfd2cc.svg">
+      <img alt="Stream Chat" width="350" height="auto" src="https://user-images.githubusercontent.com/28951144/204903022-bbaa49ca-74c2-4a8f-a05d-af8314bfd2cc.svg">
+    </picture>
   </a>
   <br>
     <h5 align="center">