Most web applications have a variety of add-on themes and plugins. Some are charged for and some are free. However, we unfortunately see themes & plugins that have issues with functionality, security vulnerabilities and bugs. It is very important to regularly update them and sometimes updates can cause problems because they can introduce incompatibilities between various components of the site.
Some themes change the website content in such a way that the content becomes dependant on the theme. If you decide to change the theme, it might not be as easy as it should be because you will also need to update the content. Visual layout editors for WordPress are mostly great, but they do significantly suffer from this issue.
Only use themes, templates & plugins from reputable sources, because it is possible for them to have issues and security vulnerabilities.