The document gives an overview on the UI and frontend architecture.
UI
QA uses Angular for the frontend and it uses Angular Material to build the UI. Angular Material is a component library and a custom styling file is used to create a theme. This can be found in src/main/frontend/custom-theme.scss in the repository.
Architecture
The frontend source code is split into three areas:
Core
Features
Shared
The core module contains singletons. The feature folder is a collection of modules that are specific to a certain part of QA. The shared module contains reusable content.
You can find more information on this type of architecture here.