When i load the add-on an input was appeared and said to enter the path, you can edit this part using the $sdir identifier that will be more easier.
Change the
set %theme.loaded $?="Type a theme name to load:"
set %theme.loaded.path %theme.dir $+ %theme.loaded
to (and delete the ON START event is unuseful)
set %theme.loaded.path $sdir($mircdir,Select the theme to load)