In case you may wish to add a specific CSS class to just the <img> tag for the header logo element, this filter can come in handy.

add_filter('blocksy:header:logo:img:class', function ($classes) {
	return array_merge(
		$classes,
		[
			'default_logo' => 'my_class',
			'transparent_state_logo' => 'my_transparent_class',
			'sticky_state_logo' => 'my_sticky_class',
			'dark_mode_logo' => 'my_dark_mode_class',
			'offcanvas_logo' => 'my_offcanvas_class'
		]
	);
});