v4: Miscellaneous color changes (#21199)

* Darken dismiss icon opacity
* Darken text on active list group items
* Darken light gray for accessibility/contrast, then update gray and dark gray to match

scss/_close.scss

@@ -5,13 +5,13 @@
   line-height: 1;
   color: $close-color;
   text-shadow: $close-text-shadow;
-  opacity: .2;
+  opacity: .5;
 
   @include hover-focus {
     color: $close-color;
     text-decoration: none;
     cursor: pointer;
-    opacity: .5;
+    opacity: .75;
   }
 }