How can I de-register ALL styles all at once? And same with Javascript?

I hope you know what you are doing. You can use the wp_print_styles and wp_print_scripts action hooks and then get the global $wp_styles and $wp_scripts object variables in their respective hooks.

The “registered” attribute lists registered scripts and the “queue” attribute lists enqueued scripts on both of the above objects.

An example code to empty the scripts and style queue.

function pm_remove_all_scripts() {
    global $wp_scripts;
    $wp_scripts->queue = array();
}
add_action('wp_print_scripts', 'pm_remove_all_scripts', 100);

function pm_remove_all_styles() {
    global $wp_styles;
    $wp_styles->queue = array();
}
add_action('wp_print_styles', 'pm_remove_all_styles', 100);