The possibilities of the theme are essentially limitless, because you can fully use PHP there. BUT! theme, as designed to display the content of the site in a certain way, not to add functionality to the site. Therefore, the capabilities of the topic are usually limited to those that are responsible for the appearance, and all the other features: voting, ratings, redirects, SEO, etc. are made to put in plugins …

Thus, the theme can:

• Responsible for the output of site content for different types of devices: monitors and smartphones. Such themes are called adaptive. Also, themes can be fixed, one-column, two-column, and so on;
• A theme can display any kind of content;
• Can specify what content will be displayed for different users;
• Can use any design elements (pictures, videos).

As with any website, the theme is not just a color scheme, layout and pretty pictures. A truly quality themes are also very functional. The functionality of the topic is in its flexible configuration. Ie it does not add anything new to the site, but fine-tuned to the taste of the owner. For example: set the background image, change the colors of elements, customizable menu and blocks with content (widgets).