1. System Preferences- Open the System Preferences menu. The icon looks like a large gear wheel.
2. Options- There are numerous options that are accessible from System Preferences. Look for General and click on it.
3. General options- At the top of the General options, you will see Appearance, where you can change the color of the windows, buttons, and menus. Directly under that is a check box and text saying Use dark menu and Dock. Click the check box and exit the System Preferences menu. You will now notice the menu bar is dark, and when you select any menus, the drop downs will have a black background. This makes the menus easier to see than that standard gray color. If you wish to revert back to normal mode, repeat these steps and uncheck the dark menu box.
Some users may prefer Dark Mode because they feel it is easier on the eyes (particularly at night). It also is an alternative option to the transparent menus that appear in Yosemite by default.