What is the difference between bootstrap.css and bootstrap-theme.css found in the bootstrap file when we download bootstrap?
bootstrap.css is the core css for BootStrap that defines all the style for various controls/components, where as
bootstrap-theme.css defines the themes (gradient/animation) for buttons,dropdown menu,navbar,progressbar,panels.
Most of the times adding
bootstrap.css is enough for bootstrap to work, but for gradient/animation, you can use
You can see the difference on http://getbootstrap.com/css/#buttons and then click on Preview theme from the affix menu (fixed menu on right side)