One overlay libraryEvery menu and dialog over a card or the board is drawn from one library. You say what an action means; the design decides how it looks.
Designs as extensionsThe grid look is only the built-in design. Write another — a flat one, say — ship it as an extension, and let people pick it.
ExtensionsAdd pages, menu entries, settings and designs to the GUI, built in or installed beside it at runtime.