The first example we're going to create will be a rather simple theme without a lot of HTML or CSS code. It's just an example to show you the most important elements in a concrete5 theme. In order to keep things simple, we'll use the Bootstrap library created by Twitter.
Bootstrap is a frontend framework that can often help you to be faster and more efficient when creating websites and applications. There are a bunch of CSS classes you can use to get better looking elements than plain HTML would produce, but it also comes with several JavaScript functions such as a carousel gallery, a drop-down navigation, and a few more. If you haven't heard of it before, you should probably have a look at their site to get a first impression about it: