I have installed the XGTK theme and the wingpanel has become white in fullscreen mode with white text. And the colors of pop-up menus like "turn off" or "wi-fi" are wrong and unreadable. I've managed to change the color of the wingpanel itself, but how can I change the colors in menu? I am using Freya
This is largely a problem in the stylesheet you're using not targeting the right style classes for Freya's wingpanel.
The easiest thing to do would be to just switch back to the default stylesheet.
If not that, the next easiest thing to do would be to upgrade to Loki. Loki's wingpanel should work much better with 3rd-party stylesheets. However you may still encounter issues if the developer of that stylesheet isn't targeting Pantheon as an environment.
Alternatively, you can try to edit the stylesheet yourself. But unless you're familiar with Gtk.CSS and using tools like Gtk Inspector this may be difficult and any updates to this stylesheet could break your changes.