How to Switch Theme in WordPress Programmatically

function set_my_custom_theme() {
    //replace your theme name
    return 'your-theme-name';
}
add_filter( 'template', 'set_my_custom_theme' );
add_filter( 'stylesheet', 'set_my_custom_theme' );

Above code explain

the template and stylesheet filters are responsible to select the activated theme for the visitors.

In line 3 of the above code, we set the returning theme name for visitors to your-theme-name which you can change to your desired theme name.

Copy this code to the PHP file of your custom WordPress plugin to use it.

Why would I want to Switch the Active WordPress Theme?

This function will be useful if you desire to display different themes to different types of users.

For example, if you want to change the displaying theme for logged-in users. Or maybe each WordPress user or user role needs different functionalities available in another theme. You can use this function to switch the WordPress theme for them.