Just about everything in the Adapt Learning Framework is a plugin. You can create custom components for rendering learning content (like questions, or graphical components, video players etc.). You can create extensions for existing components or core framework elements (e.g. the Triggered extension hides components and reveals them on button press). You can also create custom menus or themes.

Detailed docs for themes can be found here: Styling-your-course

You should probably go here: Developing-plugins