One of the new changes to the theme options in Mac OS X is the dark mode. Apple showed a dark-mode for OS X Yosemite during WWDC 2014. But the theme option was not available till a few betas were released. (Update: If you have upgraded to El Capitan, here is how to enable dark mode in El Capitan.)
It seems like the dark theme / mode is now available for Yosemite beta users. Call it the “power-saving mode” or dark theme or whatever: it looks cool. If you love light text on dark background, you might want to check this out:
How to Enable the Dark Mode in OS X Yosemite:
Step 1: Click on “System Preferences” from the dock (or click the Apple Menu → then System Preferences)
Step 2: Next, click on “General.”
Step 3: Under the Appearance option, you should see a checkbox labeled: “Use dark menu bar and Dock.” Click the checkbox (to enable it.)
That's it. Your Mac should now exhibit a dark theme. Light text on a predominantly dark background.
You'll notice that the dark theme affects only the menu bar and the dock. These turn to a grey/black from their default white/light background colors. You will have to modify the theme with other parameters to get to an all-dark theme.
Note that the dark mode is designed for low-light, work-in-the-dark environments. It's useful when you are in a dark room (or a dimly-lit room). It's also useful when you want to use less brightness (which can be mapped to less resources).
At the time of writing this, we're still in public beta so there might be some bugs using the dark theme. Report them if you can, and wait for the final release to get a stable, uniformly-working dark theme on Mac OS X Yosemite.