Fixed the header color in fly-outs for Dark mode

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66138

app/assets/stylesheets/startup/startup-dark.scss

@@ -1673,7 +1673,7 @@ body.gl-dark .nav-sidebar .fly-out-top-item a,
 body.gl-dark .nav-sidebar .fly-out-top-item.active a,
 body.gl-dark .nav-sidebar .fly-out-top-item .fly-out-top-item-container {
   background-color: #2f2a6b;
-  color: #333;
+  color: var(--black, #333);
 }
 body.gl-dark .logo-text svg {
   fill: var(--gl-text-color);

app/assets/stylesheets/themes/theme_helper.scss

@@ -185,7 +185,7 @@
       &.active a,
       .fly-out-top-item-container {
         background-color: $purple-900;
-        color: $white;
+        color: var(--black, $white);
       }
     }
   }

ee/app/assets/stylesheets/startup/startup-dark.scss

@@ -1673,7 +1673,7 @@ body.gl-dark .nav-sidebar .fly-out-top-item a,
 body.gl-dark .nav-sidebar .fly-out-top-item.active a,
 body.gl-dark .nav-sidebar .fly-out-top-item .fly-out-top-item-container {
   background-color: #2f2a6b;
-  color: #333;
+  color: var(--black, #333);
 }
 body.gl-dark .logo-text svg {
   fill: var(--gl-text-color);