Hi there.
You can set it manually with the meta tag:
<meta name="theme-color" content="#ecd96f">
You could even set different ones for light and dark theme if your website supports them:
<meta name="theme-color" content="#ecd96f" media="(prefers-color-scheme: light)">
<meta name="theme-color" content="#0b3e05" media="(prefers-color-scheme: dark)">
If I'm not wrong, Safari chooses the theme color automatically from the background-color property.
Hope that helps 🙂