See http://i.imgur.com/a2jQAwz.png The plugin appears to set its own background color, ignoring the theme. On a side-note, it also appears to be ignoring the icon theme set :(
This plugin uses Gtk3 so it requires themes that work with both Gtk2 (for the current version of the panel) and Gtk3 (for the plugin). Chances are your theme: - only supports Gtk2 only - supports both Gtk2&3 but its Gtk2 part of the theme has some special rules for xfce4-panel (e.g. dark background), which are not present in the Gtk3 part. As for the icon theme, it should work just fine, but note that the plugin uses "symbolic" icons (new in Gtk3). This is done on purpose so that the icons are always visible/readable regardless of the background color.
That seems correct. However, even most official themes from gtk-engines-xfce seem to have trouble with the plugin background. As for third-party themes, the only ones I've found working so far are Numix (only newest from github) and Greybird (same). I understand that this is a modern plugin, and I can live with Numix or Greybird, but should the plugin really break most (older) themes?