logo: use adaptive color/no background for readme

This looks nicer with dark themes. Light themes should be effectively
unchanged.

README.md

@@ -1,5 +1,5 @@
 <div align="center">
-  <img src="logo/logo_text_background.svg" width="50%">
+  <img src="logo/logo_text_adaptive_color.svg" width="50%">
 </div>
 
 ## Overview

logo/logo_text_adaptive_color.svg

@@ -1,4 +1,8 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360 120">
-  <path fill="#fff" d="M0 0h360v120H0z"/>
+  <style>
+    @media (prefers-color-scheme: dark) {
+      path { fill: #fff; }
+    }
+  </style>
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 </svg>