Bluetooth is the wireless tech that your Magic Mouse and Keyboard are using to connect to your iMac. The icon is in the upper-right hand tray area next to the clock;
it looks like a rune (because that's what it's based off of).
When you click on it you'll see the devices connected via Bluetooth and you can see what the battery percentages are.
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Go to Settings > General and there is a check box labeled "Use dark menu bar and Dock".