You can customise and extend the framework with LESS a powerful lightweight dynamic style sheet language. This allows you to set variables or to perform operations within your code to get uber creative with what you can do or to simply save on repetition and bloated CSS. You can read more about it here http://lesscss.org/ and here on Smashing Magazine
The brilliant thing with Bootstrap is that you only take the CSS and JS components that you need. You can also add in lots of custom variables also. Then you just download from a really straightforward interface and go to work on your own masterpiece.
Ask your developer have they seen this the next time they produce a dogs dinner that barely works in Firefox… Check out some beautifully crafted sites here :