Return an object as the title prop on your Tag instead of a string. Then you can add any styles you want to it.

<Tab title={<span className="text-dark-blue">Website</span>}>

The problem with your current approach is that NextUI is adding the class you've given it to an element above the one that sets the text color by default, so your text color style is overridden. Check out this documentation to get a better understanding of how it works.