fix(schematics): tree folder icons do not have enough contrast (#13462)

addresses aXe/Lighthouse a11y audit failures

Author: Splaktar

src/lib/schematics/tree/files/__path__/__name@dasherize@if-flat__/__name@dasherize__.component.__styleext__

@@ -1,4 +1,4 @@
 .type-icon {
-  color: #999;
+  color: #757575;
   margin-right: 5px;
 }