.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. 
  (With the solution provided in this commit, the theme should be applied once the terminal is restarted).

* Assuming that the theme is right next to the `.osx` script, use [`${BASH_SOURCE%/*}`](http://mywiki.wooledge.org/BashFAQ/028), instead of the hard coded `${HOME}/init/`.

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Ref: #185.