Page: Manipulate correct class in response to "theme" option change

The "theme" option should result in the manipulation of class "ui-page-theme-*",
not "ui-body-*"

Fixes #6668

js/widgets/page.js

@@ -131,7 +131,7 @@ $.widget( "mobile.page", {
 
 	_setOptions: function( o ) {
 		if ( o.theme !== undefined ) {
-			this.element.removeClass( "ui-body-" + this.options.theme ).addClass( "ui-body-" + o.theme );
+			this.element.removeClass( "ui-page-theme-" + this.options.theme ).addClass( "ui-page-theme-" + o.theme );
 		}
 
 		if ( o.contentTheme !== undefined ) {

tests/unit/page/page_core.js

@@ -51,6 +51,13 @@
 		ok( $( "#a" ).hasClass( "ui-page-theme-" + themedefault ) );
 	});
 
+	test( "setting option 'theme' on page updates classes correctly", function() {
+		$( "#a" ).page( "option", "theme", "x" );
+		deepEqual( $( "#a" ).hasClass( "ui-page-theme-x" ), true, "After setting option 'theme' to 'x', the page has the new theme class" );
+		deepEqual( $( "#a" ).hasClass( "ui-page-theme-" + themedefault ), false, "After setting option 'theme', the page does not have default theme class" );
+		$( "#a" ).page( "option", "theme", themedefault );
+	});
+
 	test( "B page has non-default theme matching its data-theme attr" , function(){
 		$( "#b" ).page();
 		var btheme = $( "#b" ).jqmData( "theme" );