Indicator plugin does not adhere to system themes. When moused over, indicator plugin always highlights itself in white/gray no matter what theme is being used. Menus for volume/music look distorted when using anything other than default theme. Linux mint 17 xfce edition, indicator plugin version 2.3.2.
Most likely you are using a theme that does not support Gtk3, or has special rules for Gtk2 panel plugins but no such rules for Gtk3 plugins. At the moment, xfce4-indicator-plugin is the only Gtk3 plugin for Xfce panel - it has to be because Unity indicators no longer support Gtk2. Try Adwaita or a new version (git) of Greybird or Numix and let me know if the problem persists.
I confirm that this bug doesn't occur with Adwaita, Greybird and Numix on Xubuntu 15.10. It appears with Bluebird, but I think this theme is no longer updated.