This is not the way to change the color. Besides, your sitemap page does not support the html language. Specifically how to change it to open your website template, you can find the navigation bar color under the color classification. Then make changes.
In shipify admin dashboard, the site map manu do not accept html code, so you can't use code to change color.
According to your requirement, if you wanna change color, it works when:
1. The Template support.
Some template support menu color changing, you can go to my shopify, admin dashboard, the template mouldify, homepage to see if it support.
2. HTML code edit.
Find a IT people to edit html code on home page manu so you can change the color.
As my dashboard in Chinese, i'm not sure if the link name was right. But i'm sure you know what i mean. LOL