“How to use Ableton Live Skins. Ableton Live Skins are files that allow you to change the colors used in Ableton Live. This page describes how to install them.
Download any skin for sonicproducer’s library: http://sonictransfer.com/ableton-live-skins/

Then, you need to find your Ableton Live Skins folder on your computer.

For Windows users
The Ableton Live Skins folder is located in Program Files\Ableton\Live\Resources\Skins.

For Mac users
The Ableton Live Skins folder is inside the Live application. Right-click (or ctrl-click) the Live application and select Show Package Contents to access the Skins.

Adding A New Skin

After you have found your Ableton Live Skins folder, you can copy any skin file into it. If you are running Live, you will need to restart it before it will see the new skin. Once you start Live, click on the Options menu, then click Preferences. Go to the Misc tab and select your skin under the Appearance section. ”

Article from http://sonictransfer.com/adding-skins-to-ableton-live.shtml