WordPress/wp-content/themes/twentytwelve/js/theme-customizer.js

/**
 * Theme Customizer enhancements for a better user experience.
 *
 * Contains handlers to make Theme Customizer preview reload changes asynchronously.
 * Things like site title, description, and background color changes.
 */

( function( $ ) {
	// Site title and description.
	wp.customize( 'blogname', function( value ) {
		value.bind( function( to ) {
			$( '.site-title a' ).text( to );
		} );
	} );
	wp.customize( 'blogdescription', function( value ) {
		value.bind( function( to ) {
			$( '.site-description' ).text( to );
		} );
	} );
	// Header text color
	wp.customize( 'header_textcolor', function( value ) {
		value.bind( function( to ) {
			$( '.site-title a, .site-description' ).css( 'color', to );
		} );
	} );

	// Hook into background color/image change and adjust body class value as needed.
	wp.customize( 'background_color', function( value ) {
		value.bind( function( to ) {
			var body = $( 'body' );

			if ( ( '#ffffff' == to || '#fff' == to ) && 'none' == body.css( 'background-image' ) )
				body.addClass( 'custom-background-white' );
			else if ( '' == to && 'none' == body.css( 'background-image' ) )
				body.addClass( 'custom-background-empty' );
			else
				body.removeClass( 'custom-background-empty custom-background-white' );
		} );
	} );
	wp.customize( 'background_image', function( value ) {
		value.bind( function( to ) {
			var body = $( 'body' );

			if ( '' != to )
				body.removeClass( 'custom-background-empty custom-background-white' );
			else if ( 'rgb(255, 255, 255)' == body.css( 'background-color' ) )
				body.addClass( 'custom-background-white' );
			else if ( 'rgb(230, 230, 230)' == body.css( 'background-color' ) && '' == _wpCustomizeSettings.values.background_color )
				body.addClass( 'custom-background-empty' );
		} );
	} );
} )( jQuery );