This panel is mostly about how things look on the screen: windows, menus, buttons, scroll bars, and fonts. (It used to be called Appearance, but General is more like—yes, you guessed it—the iPhone and the iPad.)
Two pop-up menus let you crank up or tone down OS X’s overall colorfulness:
Appearance. Choose between Blue and Graphite. Blue refers to OS X’s factory setting—bright, candy-colored progress bars, menu, and pulsing OK buttons—and those shiny red, yellow, and green buttons in the corner of every window. If you, like some graphics professionals, find all this circus-poster coloring a bit distracting, then choose Graphite, which renders all those interface elements in various shades of gray.
Use dark menu bar and Dock. New in Yosemite: a dark theme. When you turn this checkbox on, the backgrounds of your menu bar and Dock turn dark gray. Here again, Apple probably had visual artists in mind with this feature—but it looks cool and calming no matter who you are.
Highlight color. When you drag your cursor across text, its background changes color to indicate that you’ve selected it. Which color the background becomes is up to you; choose a shade from the pop-up menu. The Highlight color also affects such subtleties as the lines on the inside of a window as you drag an icon into it.
If you choose Other, the Color Picker palette appears, from which you can choose ...