A theme package contains graphics for one or many changeable components of a mobile phone’s UI. Changing a theme only affects the look and feel of the UI and not the entire phone’s functions or features.
There are many mobile operating systems which support themes as standard, such as Samsung with their Theme Store, Huawei or LG. Although Google's Android operating system doesn't support themes as standard, there are applications that provide it.
There are sites that offer customization services for phone themes