Change %x.ini in the /load_vars command to $1, then you use it like /load_vars <number>.

The whole thing can be simplified though:

Code:
alias -l tini { return $shortfn($mircdirvxd.ini) }
alias save_theme_number {
  set %x.ini $calc($ini($tini,$ini($tini,0)) + 1)
  write_vars
}
alias write_vars {
  writeini $tini %x.ini stile %stile
  writeini $tini %x.ini chiarox %chiarox
}
alias load_vars {
  set %stile $readini($tini,n,$1,stile)
  set %chiarox $readini($tini,n,$1,chiarox)
}