Themes with Twitter Bootstrap are a double-edged sword. On one hand, the themes add diversity to the visual design of bootstrap sites, which tend to rely on the default styles. You can frequently spot a bootstrap site a mile away because it features the black gradient header bar and Helvetica for the body copy. On the other hand, themes provide more options for being lazy with your site's look, as opposed to taking the time to tailor styles to your match your product/brand/content. Bootstrap is supposed to help you get a decent site up faster, but it shouldn't do all of the styling for you.