README.md: Add responsive logo for light and dark themes (#6253)

Updated logo display to support light and dark themes.

Author: masenf

README.md

@@ -1,5 +1,9 @@
 <div align="center">
-<img src="https://raw.githubusercontent.com/reflex-dev/reflex/main/docs/images/reflex.svg" alt="Reflex Logo" width="300px">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/reflex-dev/reflex/main/docs/images/reflex_light.svg">
+  <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/reflex-dev/reflex/main/docs/images/reflex_dark.svg">
+  <img alt="Reflex Logo" src="https://raw.githubusercontent.com/reflex-dev/reflex/main/docs/images/reflex.svg" width="300px">
+</picture>
 
 <hr>