Whenever Google rolls out any new update / upgrade for Android smartphones, developers over the world quickly get their hands on its source code to customize it as the way they want. And that modified OS is then called as a Custom ROM. There are many custom ROMs available for Xperia Sola but Cyanogenmod is the one which offers you lot of customization features too. Just like CM7 (which was based on Gingerbread 2.3.x), Cyanogenmod has incremented its version to 9.3 (which is based on Android 4.0.4 ICS).
