How to change the default background color white to something else in twitter bootstrap

You can overwrite Bootstraps default CSS by adding your own rules.

<style type="text/css">
   body { background: navy !important; } /* Adding !important forces the browser to overwrite the default style applied by Bootstrap */
</style>

I would not recommend changing the actual bootstrap CSS files. If you do not want to use Jako's first solution you can create a custom bootstrap style sheet with one of the available Bootstrap theme generator (Bootstrap theme generators). That way you can use 1 style sheet with all of the default Bootstrap CSS with just the one change to it that you want. With a Bootstrap theme generator you do not need to write any CSS. You only need to set the hex values for the color you want for the body (Scaffolding; bodyBackground).

You have to override the bootstrap default by being a bit more specific. Try this for a black background:

html body {
    background-color: rgba(0,0,0,1.00);

}