Yosemite Unity UI 
An Atom theme for a more native experience on OS X 10.10 Yosemite

Install
From the command line:
$ apm install yosemite-unity-ui
Alternatively, open Atom Preferences, select Themes > Search for yosemite,
then Install.
Activate the theme by selecting the Themes section of Preferences.
Replacing the Atom Icon
If you wish to replace the official Atom Icon with the following Atom-Yosemite-Icon designed by Rory McCawl just follow these steps.
More yosemite themes
- Chrome theme for Mac OS X Yosemite by @corbindavenport (github)