navbar color in Twitter Bootstrap

You can overwrite the bootstrap colors, including the .navbar-inner class, by targetting it in your own stylesheet as opposed to modifying the bootstrap.css stylesheet, like so:

.navbar-inner {
  background-color: #2c2c2c; /* fallback color, place your own */

  /* Gradients for modern browsers, replace as you see fit */
  background-image: -moz-linear-gradient(top, #333333, #222222);
  background-image: -ms-linear-gradient(top, #333333, #222222);
  background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#333333), to(#222222));
  background-image: -webkit-linear-gradient(top, #333333, #222222);
  background-image: -o-linear-gradient(top, #333333, #222222);
  background-image: linear-gradient(top, #333333, #222222);
  background-repeat: repeat-x;

  /* IE8-9 gradient filter */
  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr="#333333", endColorstr="#222222", GradientType=0);
}

You just have to modify all of those styles with your own and they will get picked up, like something like this for example, where i eliminate all gradient effects and just set a solid black background-color:

.navbar-inner {
  background-color: #000; /* background color will be black for all browsers */
  background-image: none;
  background-repeat: no-repeat;
  filter: none;
}

You can take advantage of such tools as the Colorzilla Gradient Editor and create your own gradient colors for all browsers and replace the original colors with your own.

And as i mentioned on the comments, i would not recommend you modifying the bootstrap.css stylesheet directly as all of your changes will be lost once the stylesheet gets updated (current version is ^v2.0.2) so it is preferred that you include all of your changes inside your own stylesheet, in tandem with the bootstrap.css stylesheet. But remember to overwrite all of the appropriate properties to have consistency across browsers.