Thema colur in HTML
<!-- Theme-color is use to set the background to your favorite/Branding color
Add two attribute in meta tag first 'name="theme-color"' and second 'content="Your Hexadecimal Code"'
Add Below Meta property to your HTML code in HEAD Section -->
Example:
<head>
...
<meta name="theme-color" content="#444444">
...
</head>
Ankur