If you were as confounded as I was when Redactor's default font switched to Trebuchet, install this little bugger.
Download the repo (that green button in the corner), rename the folder helvetica, and drop it in craft/plugins.
Alternatively, add it as a submodule:
$ cd path/to/your/craft/project
$ git submodule add https://github.com/AugustMiller/craft-redactor-helvetica.git craft/plugins/helvetica
…then jump into the admin and enable it!
I know, this is ridiculous. If you've got a project-specific plugin, you can basically copy the contents of the init method of HelveticaPlugin.php into your own plugin, then drop in styles.css into its resources/css directory.
🌳