Use fs.extension instead of regex

src/app/atom-theme.coffee

@@ -8,8 +8,8 @@ class AtomTheme extends Theme
     @stylesheets[stylesheetPath] = fs.read(stylesheetPath)
 
   load: ->
-    if /\.css$/.test(@path)
-      @loadStylesheet @path
+    if fs.extension(@path) is '.css'
+      @loadStylesheet(@path)
     else
       metadataPath = fs.resolveExtension(fs.join(@path, 'package'), ['cson', 'json'])
       stylesheetNames = fs.readObject(metadataPath).stylesheets