Question

How do I change the header menu (Questions, Tags, Users etc) to have no frame?

I mean actually I wish to have no background color or a white background color (and just see the text) inside the 'Questions', 'Tags', 'Users', etc boxes.

If I use the following code:

/*set the default header*/
.nav li {
        background: #ffffff;
        color: #333333;
}

/*set the selected header*/
.nav .youarehere {
        background: #ffffff;
        color: #555555;
}

/*set the hover header*/
.nav li:hover {
        background: #ffffff;
        color: #222222;
}

I get the following header menu: (everything is white: background AND Font color):

Answer 1

Extending my answer to your other question. If you want black text on white background for all tabs, just set the colours as follows This sets the hover to white on black.

.nav li a {
	background: #FFFFFF;
	color: #000000;
}

.nav .youarehere {
	background: #FFFFFF;
	color: #000000;
}

.nav li a:hover {
	color: #FFFFFF;
	background: #000000;
}