As a xfce4 user I am just making a suggestion that I believe will make the theme look a little better for xfce only. Just make the xfwm4 theme the exact same color as the top of thunar . Here is one screenshot with your xfwm4 and the one at bottom has a different one that is really close to the color at top of thunar.This is zukitwo xfwm4 at the bottom and it doesn't quite match but you can get the idea.
Like I said these are only suggestons.
As a xfce4 user I am just making a suggestion that I believe will make the theme look a little better for xfce only. Just make the xfwm4 theme the exact same color as the top of thunar . Here is one screenshot with your xfwm4 and the one at bottom has a different one that is really close to the color at top of thunar.This is zukitwo xfwm4 at the bottom and it doesn't quite match but you can get the idea. Like I said these are only suggestons.