WordPress/wp-admin/js/theme-preview.dev.js

var thickDims;
jQuery(document).ready(function($) {

	thickDims = function() {
		var tbWindow = $('#TB_window'), H = $(window).height(), W = $(window).width();

		if ( tbWindow.size() ) {
			tbWindow.width( W - 90 ).height( H - 60 );
			$('#TB_iframeContent').width( W - 90 ).height( H - 90 );
			tbWindow.css({'margin-left': '-' + parseInt((( W - 90 ) / 2),10) + 'px'});
			if ( typeof document.body.style.maxWidth != 'undefined' )
				tbWindow.css({'top':'30px','margin-top':'0'});
		};

		return $('a.thickbox-preview').each( function() {
			var href = $(this).parents('.available-theme').find('.previewlink').attr('href');
			if ( ! href ) return;
			href = href.replace(/&width=[0-9]+/g, '');
			href = href.replace(/&height=[0-9]+/g, '');
			$(this).attr( 'href', href + '&width=' + ( W - 110 ) + '&height=' + ( H - 100 ) );
		});
	};

	thickDims()
	.click( function() {
		var alink = $(this).parents('.available-theme').find('.activatelink'), url = alink.attr('href'), text = alink.html();

		if ( null == text ) text = '';
		$('#TB_title').css({'background-color':'#222','color':'#cfcfcf'});
		$('#TB_closeAjaxWindow').css({'float':'left'});
		$('#TB_ajaxWindowTitle').css({'float':'right'})
			.append('&nbsp;<a href="' + url + '" target="_top" class="tb-theme-preview-link">' + text + '</a>');

		$('#TB_iframeContent').width('100%');
		return false;
	} );

	$(window).resize( function() { thickDims() } );
});

function tb_position() {
	thickDims();
}