Volume icon does NOT change from a black icon to a white icon
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Right click top panel, "Panel", "Panel Preferences...", "Appearance" tab, set "Style:" to Solid Color, click "Color:" box. Enter #000000 for "Color name:". Then click OK. The bar color changes to black, but the text color does not automatically change to white. Maybe there should be a "Text" label, with "Type:" and "Color:".
a) "xfce4-pulseaudio-plugin" volume icon does NOT change from a black icon to a white icon.
Click "Apps", "Settings", "Appearance", click "Xfce-dusk". There is a white border around the "xfce4-pulseaudio-plugin" volume icon. The volume icon should change to a white icon.