<!-- for a singular color regardless of theme -->
<meta name="theme-color" content="#ffffff">
<!--Different ones for light and dark theme if your website supports them: -->
<meta name="theme-color" content="#ffffff" media="(prefers-color-scheme: light)">
<meta name="theme-color" content="#000000" media="(prefers-color-scheme: dark)">
<!--source: https://developer.apple.com/forums/thread/683403 -->