Ok not sure about that, but a fact that Reaper decides
which theme uses which images seems a bit pointless,
you would want a theme to be a complete package, and
maybe let an option for user to override the default
path, but the default should be the same folder name
or a path definition in .ReaperTheme file, it's only
logical.
EDIT: false alarm, sorry, haven't been themeing
for a while and default v4 theme has a path definition called
"build" instead of "Default_4.0", thus it doesn't work
by default. See
this post.