WordPress enables mobile zooming by default, but sometimes Theme disabled the zoom in. All we need to do is to change it back to the default setting. I will take the attitude theme for example.

1.  First of all, it is always a good practice to create a child theme before modifying a theme, so we can keep our modification while updating the themes, find the modification easily. See this page to learn how to create a child theme.http://codex.wordpress.org/Child_Themes.
2. Then copy the header.php from the theme_name folder to the child_theme folder.After this we will do some modification on the header.php in the child_theme folder.
3. Open the header.php file in the child_theme folder and find the code like this

 

/**
		 * attitude_meta hook
		 */
		do_action( 'attitude_meta' );

and change it into:

?>
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<title><?php bloginfo('name'); ?> <?php wp_title(); ?></title>
	<?php