XfApplet does not change it's background color at all - even if you change the whole desktop's theme. I found this behaviour using the Xfce dawn theme with XfApplet present in the panel (containing MintMenu).
Mass-reassign all bugs from florian@ to goodies-dev@, thanks for the maintenance work! (and sorry for the bugmail spam..)
Closing all xfce4-xfapplet-plugin (archived/deprecated/unmaintained project).