docs(core): update banner image (nrwl#12373)

Author: bcabanes

README.md

@@ -1,5 +1,9 @@
-<p style="text-align: center;"><img src="https://raw.githubusercontent.com/nrwl/nx/master/images/nx.png" 
-width="100%" alt="Nx - Smart, Fast and Extensible Build System"></p>
+<p style="text-align: center;">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/nrwl/nx/master/images/nx-light.svg">
+    <img alt="Nx - Smart, Fast and Extensible Build System" src="https://raw.githubusercontent.com/nrwl/nx/master/images/nx-dark.svg" width="100%">
+  </picture>
+</p>
 
 <div style="text-align: center;">