There are three different UI toolkits you can use to development for KDE Application or the Plasma Workspace (Due to technical limitations, the toolkits have slight visual differences, but the recommended interaction patterns to be used are consistent regardless of the toolkit).
There are two UI toolkits that can be used to develop KDE Applications:
Kirigami is KDE’s lightweight user interface framework for mobile and convergent applications. It allows Qt developers to easily create applications that run on most major mobile and desktop platforms without modification (though adapted user interfaces for different form-factors are supported and recommended for optimal user experience). It extends the touch-friendly Qt Quick Controls with larger application building blocks.
Use Qt Widgets only if you plan to develop a desktop-only application with a complex UI, like KDevelop.
Plasma is built on widgets, allowing you to move, mix, add, and remove just about everything to perfect your personal workflow. Use plasma components v3 to develop widgets for the Plasma Mobile and Plasma Desktop workspace.
The KDE HIG defines a set of common components which are independent of any device type.
Workspace: The top-level container of the whole user interface. Often called “desktop”, “home screen”, or “shell”, it shows the wallpaper and allows users to add widgets, app launchers, files or folders.
Application Launcher: Provides an overview of installed applications and allows the user to launch one of them.
Application Shortcuts: Provides quick access to frequently-used applications.
Active Application Overview: Provides an overview of the active applications that are directly used by the user.
Workspace Tools: Provides quick access to functionality integrated into the workspace that is both highly visible to the user and frequently changed, like enabling/disabling WiFi and Bluetooth, or whether or not to show notifications.
Application-Workspace Interaction: Displays information about each application’s windows, and provides ways to move or close them and change how they run within the workspace.
Application: The top-level container of a single application.
Application Tools: Provides access to an application’s commonly-used functionality in an always-accessible toolbar or menubar. These tools should not change depending on what the application is displaying.
Application Content: The actual content of an application. This depends on the application itself, but conformance to the KDE HIG should make it easier to allow convergence for this component. This part of the application can also contain contextually-appropriate tools that operate directly on the active or selected content.
There are three different kinds of themes influencing the the look-and-feel of KDE applications and the Plasma workspace.
The default for all there of them is Breeze.
Only Breeze, Breeze dark, Breeze Light, Breeze Highcontrast are covered by the HIG, all other themes are not covered.