You need to customize the css. Create your own css file and add it to index.html _after_ the style.css.
In that css you can overwrite any styles, like the top bar color. I suggest you use a browser that has decent element/style inspector, like Chrome. There you can right click the element, choose "Inspect element" and you can see what styles are defined and where. Of course you have to understand DOM hierarchy and CSS rules, but I suppose that goes without saying if you want to customize a html page.
The color comes from CSS selector ".navbar-inverse .navbar-inner" so that's what you need to overwrite.