We got a lot of Cyanogenmod Builds ever since Google made Android Lollipop Official, but all of them including the Official Version of Cyanogenmod 12 which launched not that long ago missed one crucial feature; The Theme Engine. Cyanogen has fixed this issue and they have finally added it in to the latest build of Cyanogenmod 12.
Using the Theme Engine anyone can easily apply system-wide themes to Rom, and change the looks of the interface.It was a major part of Cyanogenmod 11, where a lot of people used Lollipop based Roms on their devices to make it look and feel like Android Lollipop.
At this point, according to Cyanogenmod there are a few things to consider. Some of the CM11 themes are not compatible with CM12, while some might be partially compatible. All the Theme makers that used the Holo Style should update their themes to Lollipop to give some fresh look. Some themes may work fine as well.
One more thing to point out, According to Cyanogen, Some older themes won’t work at all due to the mixup of codes. Themes using T-Mobile’s theme engine are also not supported.
The Themes made for Android Lollipop based Cyanogenmod 12 won’t be compatible with Cyanogenmod 11 as well, so the theme makers shuld use the new version to make Themes with design ques from Android Lollipop to make it future proof.
Source – Clark Scheff