Design is sometimes a controversial topic and some elements in Nextcloud don’t work ideal for everybody. A good way to handle this is to define UI standards and set of global CSS classes which can be used by “theming” apps to change visual aspects of the UI.
So far there is CustomCSS which allows to add custom CSS rules. But using this requires detailed knowledge of the Nextcloud UI and changes done this way will likely break Nextcloud updates if the structure of the UI will change.
Is there any kind of documentation which covers the structure of the Nexcloud UI and what standard elements should be used for specific parts? The development tutorials cover the technical structure of an interface only app but don’t contain any recommendation what UI elements to use for which purpose.