Change the Action Center Icon on Your Mac to Whatever You Want
The Notification Center icon in Yosemite is pretty bland. If you want to spice things up a bit, iDownloadBlog will show you how to replace it with a new icon.
Basically, you are just replacing the system file here, so be sure to back up the PDF file you are replacing before doing anything. Otherwise, any PDF of a suitable size will work here, but iDownloadBlog includes a link with the moon icon in the video above in your post if you want to.
How to Change the Action Center Icon in the Menu Bar on Mac | iDownloadBlog