The CyanogenMod team introduced a theme manager in CM7, allowing users to change the look and feel of Android 2.3 by installing custom themes. Now the theme manager has been updated for Android 4.0.
It’s available in the latest nightly builds of CyanogenMod 9, which is still in pre-release testing.
You can try out the theme manager by downloading a nightly build of CyangoenMod for your device dated 5/07/2012 or later. Builds aren’t available for all devices yet.
There also aren’t actually any themes available for CyanogenMod 9 yet — but that should change pretty soon.
Update: You can find a couple of themes at reddit.
The latest nightlies also add a new feature that lets you add custom shortcuts to your lock screen. This lets you launch any app without first swiping to unlock your phone or tablet.