One thing I would really like to see in the Xfwm4 theme format is a way to mix GTK+ colors in XPM images.
I would like to add nice, smooth edges in themes, in a way that supports GTK+ colors.
AFAIK, layering PNG images with alpha transparency on top of the XPM images does not allow for color pickup in the PNG images themselves, and if one wants to truly respect the GTK+ theme colors, one can only use that technique to add somewhat subtle nuances. Especially considering that users may choose to use everything from a dark/inversed GTK+ theme, to a high contrast theme with white background color.
What I propose is to allow something like this:
"# c #4B58A2 s mix(0.82, active_color_2, active_shadow_2)",
It would work like in GTK+ themes:
"mix (factor, color1, color2)
Computes a new color by mixing color1 and color2. The factor determines how close the new color is to color1. A factor of 1.0 gives pure color1, a factor of 0.0 gives pure color2."
--Extract from http://library.gnome.org/devel/gtk/stable/gtk-Resource-Files.html
If this were to be implemented, the shade color feature from GTK+ themes could be considered for implementation as well.
I don't think such a syntax would be understood by standard XPM.
But nevertheless, if you provide a clear and well written patch for that, I'll consider the feature.