A theme is responsible for the look of your website. concrete5 ships with four default themes. We can choose between those themes. You can see them when you navigate to Dashboard and then click on Themes. You can see all the default themes on the following screenshot:
A theme isn't restricted to a certain layout structure such as a blog. concrete5 themes can be built using any HTML and CSS code you want. You can even create your JavaScript files on the fly. You therefore find lots of different layouts in the marketplace and there are pretty much no limits, so you can build whatever you like.
You can activate one of the default themes by clicking on Activate. Confirm the activation on the next screen and your website changes its layout immediately. If you're working on an active site, you might want to hit Preview first to see how your page is going to look with the new theme.