themes-wordpress/blockbase/sass/base/_header.scss

.wp-site-blocks .site-header {
	justify-content: start;
	overflow: inherit;
	padding-top: var(--wp--custom--gap--vertical);

	.wp-block-navigation {
		flex-grow: 1;
		margin-top: 0;
	}
	// The blockGap is used HORIZONTALLY when the viewport is LARGE (in which case the value defined in theme.json is appropriate)
	// It needs to be a different value then when the viewport is SMALL and the gap is used VERTICALLY
	@include break-small-only() {
		gap: 2px;
	}
	.wp-block-site-logo {

		@include break-small-only(){
			flex-basis: 100%;
			margin-bottom: calc( 2 * var(--wp--custom--gap--vertical));
			margin-top: var(--wp--custom--gap--vertical);
			text-align: center;
		}

		a > img {
			height: 64px;
			width: auto;
		}
	}

	.wp-block-site-tagline {
		@include break-small-only() {
			flex-basis: 100%;
			order: 10;
		}
	}

	.wp-block-navigation__responsive-container-content {
		// Needed until https://github.com/WordPress/gutenberg/issues/35549 is fixed.
		display: flex;
		gap: var( --wp--style--block-gap, 2em );
	}
}