there is no Accessibility choice in the System Preferences to allow a change to transparency
When I first installed Yosemite I chose to reduce the Transparency (I think?) but I have tried going back to check but am unable to find the Accessibility icon shown below next to Time Capsule..
There is an Accessibility icon in Security & Privacy, but this does not possess the options for Transparency etc. as shown in this screen capture. I am totally dumbfounded!
iMac, OS X Yosemite (10.10.1), null