Hello,

This line is the part of the top browser panel it does not relate to our theme and it is not possible to change with the theme or WordPress. If your Windows has been configured for the white color scheme, the panel would be white with a dark border: https://prnt.sc/1lo6tho

When you chose the dark color scheme for Windows, the panel would be dark and the border would be white https://prnt.sc/1lo71pu

You can see the same viewing other sites when Windows is set for the dark color scheme: https://gyazo.com/fa75cbe3c32f99d5394fb8e7b569ed82

If you have any questions please feel free to contact us.

Best Regards