How get Themes list via REST api?

You can write your own endpoint and use wp_get_themes to get a list of themes via that. Here is a simple one:

add_action( 'rest_api_init', function () {
    //Path to rest endpoint
    register_rest_route( 'theme_API/v1', '/get_theme_list/', array(
            'methods' => 'GET', 
            'callback' => 'theme_list_function' 
    ) );
});
// Our function to get the themes
function theme_list_function(){
    // Get a list of themes
    $list = wp_get_themes();
    // Return the value
    return $list;
}

Now you can get a list of your themes by accessing http://example.com/wp-json/theme_API/v1/get_theme_list.

I wouldn’t suggest activating/deactivating themes via API. It can totally mess up things, such as activated widgets.