Edit: Updated build, and saved file.
v1.6.0 Build 14491.1150947 (no mods, integrity verified)
Although all factions share a common Empire-Themed background art on both technology and skill tree windows, Wood Elves are unique in that they feature an Asrai-Themed art on the technology tree background. They however, still use the common background art on lord/hero skill tree window.
Wood Elves/Argwylon character skills window should have the same Asrai themed unique art in the background as technology tree window does.