Within Totara, a theme can change the way an instance looks and feels. One of the ways it can do it is by overriding mustache templates.
Overriding mustache templaes
You should override a mustache template when layout cannot be altered enough via CSS.
Mustache templates go in the templates folder within your theme. We recommend that templates that are called from within your theme to be placed in the base directory. All others should be placed in a subdirectory with the frankensense name (eg. if you are overriding a template found in mod/foo/templates/bar.mustache, then the template should be stored in theme/<themename>/templates/mod_foo/bar.mustache).