Theme Base Guide
Another handy capability is the theme configuration: As a theme developer you can define variables which can be configured by the shop owner in the Administration. Those variables are accessible in your theme and let you implement powerful features.
Basically a theme can be an app/plugin that aims at changing the visual appearance of the Storefront. To be better understand the difference among Plugins and Apps vs Themes, refer to the corresponding article.
Now that you know what you can do with themes, the next steps would be to create themes.