This is what controls the nav tabs (colors, fonts, background (gradient), borders, border radius'. text shadows, etc.
#nav a {background:#000;
background:-moz-linear-gradient(top, #454545, #000);
background:-o-linear-gradient(top, #454545, #000);
background:-webkit-gradient(linear, left top, left bottom, from(#454545), to(#000));
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr='#454545', endColorstr='#000000' );
-ms-filter:"progid:DXImageTransform.Microsoft.gradient(startColorstr=#454545, endColorstr=#000000)";
-webkit-background-clip:padding-box;
border:1px solid #000;
-moz-border-radius:3px;
-ms-border-radius:3px;
-o-border-radius:3px;
-webkit-border-radius:3px;
border-radius:3px;
color:#fff;
display:block;
font-size:11px;
font-weight:bold;
line-height:20px;
margin:0 1px 1px 0;
padding:4px 10px;
position:relative;
text-decoration:none;
text-shadow:0 1px 1px #000;
text-transform:uppercase;
}
The problem is that in the past if you tried to edit and include all the background declarations with all the different browser-specific stuff, wordpress would strip it out because the CSS tidy CSS program they are using was not up to date. The "filter" and
"-ms-filter" stuff is Internet Explorer specific stuff.