Add CSS class indicating platform on body

src/atom.coffee

@@ -44,6 +44,7 @@ class Atom
 
   initialize: ->
     @unsubscribe()
+    @setBodyPlatformClass()
 
     {devMode, resourcePath} = atom.getLoadSettings()
     configDirPath = @getConfigDirPath()
@@ -68,6 +69,11 @@ class Atom
     @pasteboard = new Pasteboard()
     @syntax = deserialize(@getWindowState('syntax')) ? new Syntax()
 
+  # Private:
+  setBodyPlatformClass: ->
+    bodyEl = document.getElementsByTagName('body')[0]
+    bodyEl.classList.add("platform-#{process.platform}")
+
   getCurrentWindow: ->
     remote.getCurrentWindow()