.osx: Fix problem with setting the Terminal theme

* `Terminal.app` needs to be "informed" about the theme change, otherwise it will just overwrite the new settings (Ref: #185).
  (Note: with the solution provided in this patch, the theme should be applied once the terminal is restarted).

* Assuming that the theme is always right next to the `.osx` script, use [`${BASH_SOURCE%/*}`](http://mywiki.wooledge.org/BashFAQ/028), instead of the hard coded `${HOME}/init/`.