I have to admit I don't really like the new design of OS X Yosemite, and I would like to get the good old Mavericks theme on the current Yosemite install.
Is this possible?
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You can swap icons with LiteIcon but not the overall theme, as far as I know.
You can disable transparency by going to System Preferences > Accessibility > Display > Reduce Transparency
That may help you a little, in addition to changing back the icons as previously answered.