Author: Rajesh Taneja
MDL-27862 Themes: Added button to un-select theme for devices, except default device
http://git.moodle.org/gw?p=moodle.git;a=commit;h=2942426ff47bdba4f374d4db7a53bf769c4d0188
http://github.com/moodle/moodle/commit/2942426ff47bdba4f374d4db7a53bf769c4d0188
+ 2.1 en [unsettheme,core_admin]