Comment 3 for bug 828392

In the beginning of /usr/share/themes/Ambiance/gtk-3.0/apps/gnome-panel.css:

PanelWidget, PanelApplet, PanelToplevel {
    background-image: -gtk-gradient (linear, left top, left bottom,
                                     from (shade (@dark_bg_color, 1.5)),
                                     to (shade (@dark_bg_color, 1.05)));
    <...>
}

I don't see any errors here, so maybe it's Gnome Panel issue? I don't know any other theme that works and uses gradients.

BTW, an easy workaround is to go to panel properties -> "Background" -> set to "Image" -> select "/usr/share/themes/Ambiance/gtk-2.0/apps/img/panel.png" file.