Tsk not very enticing going through all those textures. I went through quite a bunch in hopes of finding it, but did not. As you say, it's probably considered non-UI so it won't be there, sadly.
Btw, the border that surrounds stuff is often a shader effect rather than a texture, so I wouldn't expect to find a texture that directly affects it. But that Mark ability is clearly just a texture stuffed in 3D, like other UI elements.
Oh well. Maybe the devs read this and get encouraged to open up more assets for our modding.
