After downloading a complete pack of bootstrap 3 from http://getbootstrap.com, I noticed that there is a separate css file for theme. How to make use of it? Please explain?<
For an example of the css styles have a look at: http://getbootstrap.com/examples/theme/
If you want to see how the example looks without the bootstrap-theme.css file open up your browser developer tools and delete the link from the of the example and then you can compare it.
I know this is an old question but posted it just in case anyone is looking for an example of how it looks like I was.
Update
bootstrap.css = main css framework (grids, basic styles, etc)
bootstrap-theme.css = extended styling (3D buttons, gradients etc). This file is optional and does not effect the functionality of bootstrap at all, it only enhances the appearance.
Update 2
With the release of v3.2.0 Bootstrap have added an option to view the theme css on the doc pages. If you go to one of the doc pages (css, components, javascript) you should see a "Preview theme" link at the bottom of the side nav which you can use to turn the theme css on and off.