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=f5ee52a7fe905960722fa28f076ff0d1b0108eb5
http://github.com/moodle/moodle/commit/f5ee52a7fe905960722fa28f076ff0d1b0108eb5
+ 2.2 en [unsettheme,core_admin]