Light/dark readme images (#438)

* New image

* Light/dark images

* Update README.md

README.md

@@ -3,7 +3,10 @@
 
 Secretive is an app for storing and managing SSH keys in the Secure Enclave. It is inspired by the [sekey project](https://github.com/sekey/sekey), but rewritten in Swift with no external dependencies and with a handy native management app.
 
-<img src="/.github/readme/app.png" alt="Screenshot of Secretive" width="600">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="/.github/readme/app-dark.png">
+  <img src="/.github/readme/app-light.png" alt="Screenshot of Secretive" width="600">
+</picture>
 
 
 ## Why?