Change color of balloons and text in Message.app In previous versions of Message.app bundled with Mac OS X, we could change the font and color of the message balloons. In Yosemite I no longer can find such options. 

Am I simply failing to find the options? Is such customizing no longer allowed? If not allowed, is there any easy hacking tricks to get the same effect?
In OSX Yosemite colors are blue (for iMessages) and green (for SMS). There is no longer the ability to change these; Macs now look like iOS devices.

Hard way: (backup first!)

For Yosemite modify CSS here:

/System/Library/Messages/PlugIns/Balloons.transcriptstyle/Contents/Resources/balloons-modern.css


For El Captain it's now located in:

/System/Library/PrivateFrameworks/SocialUI.framework/Versions/A/Resources


But one needs to either disable rootless mode or to use a union file system mount to map the file over without rootless.

Hope this will help.