Fix hidden trial mode link

This link wasn’t styled, therefore it had, by default, the same colour
as its background (blue).

This commit explicitly sets it to be white, so it is visible against its
background.

app/assets/stylesheets/components/banner.scss

@@ -71,7 +71,27 @@
   }
 
   &-action {
+    
+    display: block;
     text-align: right;
+    float: right;
+
+    &:link,
+    &:visited {
+      color: $white;
+    }
+
+    &:hover,
+    &:active {
+      color: $light-blue-25;
+    }
+
+    &:active,
+    &:focus {
+      background-color: $yellow;
+      color: $govuk-blue;
+    }
+
   }
 
 }