The “Convert” button appears on both theme and component pages. It will allow you to convert a theme to a component or vice versa through the admin interface. Most Discourse admins will never need to use it. Themes and components found at #theme already include the correct designation.
This should be covered in our Beginner's guide to using Discourse Themes, but the basic idea is:
- Themes are considered to be the baseline of your Discourse site’s styling. Often sites offer multiple, user-selectable themes.
- Components are meant to be added to themes to extend style and/or functionality beyond your basic theme(s).