You can use the built in function wp_is_mobile()

Override a style with wp_enqueue_style

add_action('wp_enqueue_scripts', 'my_mobile_styles');
function my_mobile_styles(){
    if(wp_is_mobile()){
        wp_register_style( 'my-mobile-style', 'URL_to_stylesheet', array('id-of-main-stylesheet') );
        wp_enqueue_style( 'my-mobile-style' );
    }
}

Switch themes by filters

You can use the stylesheet and template filters to alter which theme WordPress will render.

# Parent Theme
add_filter( 'template', 'my_mobile_template', 99999, 1);
function my_mobile_template($template){
    if(wp_is_mobile())
        return 'mobile_template_dir_name';
    return $template;
}

# Parent or Child Theme (if applicable)
add_filter( 'stylesheet', 'my_mobile_stylesheet', 999999, 1);
function my_mobile_stylesheet($stylesheet){
    if(wp_is_mobile())
        return 'mobile_stylesheet_dir_name';
    return $stylesheet;
}

Isolating to a mobile user-agent

You can further extend this by using the following extracted from wp_is_mobile() within your conditional IF statement brackets.

$is_ios = strpos($_SERVER['HTTP_USER_AGENT'], 'Mobile');
$is_android = strpos($_SERVER['HTTP_USER_AGENT'], 'Android');
$is_silk = strpos($_SERVER['HTTP_USER_AGENT'], 'Silk/');
$is_kindle = strpos($_SERVER['HTTP_USER_AGENT'], 'Kindle');
$is_blackberry = strpos($_SERVER['HTTP_USER_AGENT'], 'BlackBerry');
$is_opera_mini = strpos($_SERVER['HTTP_USER_AGENT'], 'Opera Mini');

From there, you can enqueue styles that override your main theme.

Caveat

It’s important to note there is a function called switch_theme() which is not to be used for this case. It literally changes the theme and stylesheet location in the database. switch_theme() would change your theme to the mobile version for all users whenever a mobile user-agent is detected.

Testing

I typically use some browser extensions that alter the browser user-agent definition for testing something like this. Here are some links to the ones I’ve used:

• Chrome – User Agent Switcher
• Firefox – User Agent Switcher

Hope this helps you out!