What is the difference between bootstrap.css and bootstrap-theme.css?
Solution 1:
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 bootstrap-theme.css.
You can see the difference on https://getbootstrap.com/docs/3.4/css/#buttons and then click on "Preview theme" from the very bottom of the menu on the right-hand side of the page.