make + icon grey600 for proper icon coloring dark/light

Fixes #6647

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

src/main/res/drawable/ic_plus.xml

@@ -20,5 +20,5 @@
     android:width="24dp"
     android:viewportWidth="24"
     android:viewportHeight="24">
-    <path android:fillColor="#000" android:pathData="M19,13H13V19H11V13H5V11H11V5H13V11H19V13Z" />
+    <path android:fillColor="#757575" android:pathData="M19,13H13V19H11V13H5V11H11V5H13V11H19V13Z" />
 </vector>