How can I change the aqua theme/skin to a dark one in Yosemite? I'm referring to the window title bars, scroll bars, buttons, etc (the cocoa aqua ui theme?). Everything is grayish by default, but i'd like a dark theme.
If you're still looking for a method to darken your UI in Yosemite, download Flavours 2 and choose the "Smooth Criminal" flavour.
Flavours 2 is still in Beta, but most things look great with this skin.
Go to System Preferences → General → Right under Appearance check "Use dark menu bar and dock"