I love the colour of the page 'bleeding' into the 'tab area' - providing they can maintain a decent level of contrast. It looks really nice in the provided example. Of course, I hope they've used `<meta name="theme-color"...` instead of the background colour so pages with a white background and a black header don't end up with white chrome.
Safari wouldn't apply the theme-color if it makes the UI inaccessible (it has a very few narrow range of color that it won't apply). Also, if the tag is not specified, Safari would not blend the website content into the tab bar.