Header logo with tooltip?

Hello Karsten,
no, it’s functions.php
In original functions.php locate alx_site_title() – line 320 or close.
Since you uploaded a .png image, you should override it in child theme functions.php with something like this:

function alx_site_title() {
		// Text or image?
		if ( ot_get_option('custom-logo') ) {
			$logo = '<img src="'.ot_get_option('custom-logo').'" alt="'.get_bloginfo('name').'" title="Hello World" >';
		} else {
			$logo = get_bloginfo('name');
		}

		$link = '<a href="'.home_url('/').'" rel="home">'.$logo.'</a>';

		if ( is_front_page() || is_home() ) {
			$sitename = '<h1 class="site-title">'.$link.'</h1>'."\n";
		} else {
			$sitename = '<p class="site-title">'.$link.'</p>'."\n";
		}

		return $sitename;
}

It’s “Hello World” + this is “quick hack”, probably can be shortened.
Regards,
Mike