Update charcoal theme colors

app/assets/stylesheets/framework/gitlab-theme.scss

@@ -89,8 +89,11 @@
   }
 }
-$theme-blue: #2980b9;
 $theme-charcoal: #3d454d;
+$theme-charcoal-dark: #383f45;
+$theme-charcoal-text: #b9bbbe;
+
+$theme-blue: #2980b9;
 $theme-graphite: #666;
 $theme-gray: #373737;
 $theme-green: #019875;
@@ -102,7 +105,7 @@ body {
   }
   &.ui_charcoal {
-    @include gitlab-theme(#d6d7d9, #485157, $theme-charcoal, #353b41);
+    @include gitlab-theme($theme-charcoal-text, #485157, $theme-charcoal, $theme-charcoal-dark);
   }
   &.ui_graphite {