If you wish to use your own stylesheets instead of the ones that are automatically added to the posts where you’ve inserted a Tiled Gallery, or a gallery using Carousel, you can add the following code to your theme’s functions.php file, or to a functionality plugin.

To remove Tiled Galleries’s stylesheet:

function tweakjp_rm_tiledcss(){
    wp_dequeue_style( 'tiled-gallery' );
}
add_action( 'wp_footer', 'tweakjp_rm_tiledcss' );

To remove Carousel’s stylesheet:

function changejp_dequeue_styles() {
    wp_dequeue_style( 'jetpack-carousel' );
}
add_action( 'post_gallery', 'changejp_dequeue_styles', 1001 );