BUG: Gnome-Panels Doesn’t Handle Background Images Or Transparency Correctly.
PROBLEM: You are trying to use a custom color (with transparency or without) or a custom image (with transparency or without), but some of the applets don’t take the customization, and the transparency may or may not be present.
REASON1: The theme you are using has specified a bg_pixmap for the panel. When this is set, most (if not all) applets use this value and don’t bother checking if the user has over-ridden this property.
RESULT1: Empty spaces in the panel take to the user setting, but applets will stick to the gtkrc-supplied setting.
REASON2: Image transparency for panels does not work when the image is specified in the gtkrc file with the bg_pixmap value.
RESULT 2: The panel appears solid even when GIMP shows a transparency.
SOLUTION: Comment out the offending line in the gtkrc file and use the image in the panel properties (Right-click > Properties > Background). Transparency will work as intended here.