themes-wordpress/blockbase/sass/blocks/_navigation.scss

// See https://github.com/WordPress/gutenberg/issues/34648
.wp-block-navigation {
	// See https://github.com/WordPress/gutenberg/issues/34648
	.has-child .wp-block-navigation__submenu-container {
		background-color: var(--wp--custom--navigation--submenu--color--background);
		border: var(--wp--custom--navigation--submenu--border--width) var(--wp--custom--navigation--submenu--border--style) var(--wp--custom--navigation--submenu--border--color);

		a {
			color: var(--wp--custom--navigation--submenu--color--text);
		}
	}
	&.is-responsive {
		&:not(.has-background) .wp-block-navigation__responsive-container.is-menu-open {
			background-color: var(--wp--custom--color--background);
			color: var(--wp--custom--color--foreground);
		}
		ul.wp-block-social-links {
			margin: 0;
			gap: var(--wp--custom--gap--baseline);
		}
	}
}

.wp-block-navigation.is-style-blockbase-navigation-improved-responsive {
	&.is-responsive .is-menu-open {
		font-size: var(--wp--preset--font-size--medium);

		.wp-block-navigation__container {
			row-gap: 0.5rem;
			align-items: flex-start;
			flex: unset;
			padding-bottom: 0;
		}

		.wp-block-navigation-item {
			align-items: flex-start;
		}
		.wp-block-navigation__responsive-container-content .has-child { // Needed for specificity to beat the navigation block CSS
			.wp-block-navigation__submenu-container {
				font-size: var(--wp--preset--font-size--normal);
				padding-bottom: 0;
				padding-left: var(--wp--custom--gap--horizontal);
				padding-top: 0.5rem;
				row-gap: 0.5rem;
			}
		}

		ul.wp-block-social-links {
			justify-content: flex-start;
		}
	}
}