Time Out supports two kinds of themes: Local and Remote.
Remote themes are simply a URL to an online website. Any website can be added as a remote theme; all you need is the URL and some basic information about it. Some sites are more suitable for break themes than others, of course.
Local themes are a little more complex: they are web pages hosted on your computer. They could be as simple as a single static page with some text or an image, or a full web app, running locally.