Я только что перепроверил на голой теме, написаной на underscores - стили гутенберга подключаются на уровне ядра, лежат по такому пути /wp-includes/css/dist/block-library/style.min.css.
Вот выдержка из документации:
#Default block styles
Core blocks include default structural styles. These are loaded in both the editor and the front end by default. An example of these styles is the CSS that powers the columns block. Without these rules, the block would result in a broken layout containing no columns at all.
The block editor allows themes to opt-in to slightly more opinionated styles for the front end. An example of these styles is the default color bar to the left of blockquotes. If you’d like to use these opinionated styles in your theme, add theme support for wp-block-styles:
add_theme_support( 'wp-block-styles' );
https://developer.wordpress.org/block-editor/devel...
Ищите у себя где отключено, возможно с помощью подобного кода в функциях:
// remove Gutenberg styles
function remove_gutenberg_styles() {
wp_dequeue_style( 'wp-block-library' );
}
add_action( 'wp_enqueue_scripts', 'remove_gutenberg_styles', 100 );