use semi-transparent gray instead of black

Author: mmalerba

src/material-examples/cdk-virtual-scroll-context/cdk-virtual-scroll-context-example.css

@@ -9,5 +9,5 @@
 }
 
 .example-alternate {
-  background: rgba(0, 0, 0, .2);
+  background: rgba(127, 127, 127, 0.3);
 }