Make focus colour pass WCAG

The default combination of blue on orange is not easy to read.

app/assets/stylesheets/components/navigation.scss

@@ -29,6 +29,7 @@
         border-left: 10px solid $yellow;
         border-right: 3px solid $yellow;
         right: -3px;
+        color: $text-colour;
       }
 
     }
@@ -53,6 +54,10 @@
       text-decoration: underline;
     }
 
+    &:focus {
+      color: $text-colour;
+    }
+
   }
 
 }