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=bff67f49046e6102ee277bb710b6113fa8d4ace4
http://github.com/moodle/moodle/commit/bff67f49046e6102ee277bb710b6113fa8d4ace4
+ 2.3 en [unsettheme,core_admin]