Decker has a range of primitive UI widgets like buttons, fields, sliders, spinners, and grids. Decker's font editor[0] is a moderately complex application which uses a variety of widgets.
It's also possible to build new widgets with arbitrary appearances and behaviors like date pickers, radio buttons, or tab-bars via "Contraptions"[1]. The community forum has a collection[2] of Contraptions to suit various applications; sharing or re-using them across decks is as simple as copying and pasting a widget.
[0] https://beyondloom.com/decker/fonts.html#font