Late model MacBook Air 13
From the Obsidian Menu Bar page:

To give the Clock, Battery and Fast user switching menu items readable
text, copy WhiteUIServer.menu by Alex Zielenski to somewhere on your
harddrive and open it. Also add it to System Preferences&gt; Users &amp; Groups&gt;
Login Items to keep the text tray.


"Somewhere" on your hard disk could be, say, in your Documents folder. Basically any place that you know where it is and are unlikely to muck with it later, e.g., move it. When you've placed it where you want it to be, double click it.

Last, add it to your account's login items: http://support.apple.com/kb/ht2602

That article is a bit old, but the basics are the same in later versions of OS X.