Your function should be:
function my_theme_sty() {
wp_enqueue_style( 'bootstrap', get_template_directory_uri().'/assets/css/bootstrap.css');
}
add_action( 'wp_enqueue_scripts', 'my_theme_sty' );
Your function should be:
function my_theme_sty() {
wp_enqueue_style( 'bootstrap', get_template_directory_uri().'/assets/css/bootstrap.css');
}
add_action( 'wp_enqueue_scripts', 'my_theme_sty' );