So, even with background color set like this:
body, html {
background: #000;
}
When I scroll to the bottom of the page on IOS Safari, there is this "elastic part" which reveals a white background which is not part of the design of my website. Similar thing happens on header of the page when I try to refresh:

It has been fixed on CNN.com, but I can't figure out how.

According to this question, you could try the following:
<meta name="theme-color" content="#f6f8fa" media="(prefers-color-scheme: light)">
<meta name="theme-color" content="#161b22" media="(prefers-color-scheme: dark)">
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