Leonardo UI was introduced in Qlik Sense 3.0 as an open source project of Qlik's native styling components and look and feel. It's a user interface component framework, designed to be used as a complement to other frameworks (for example, bootstrap). It is non-obtrusive in the sense that it does not conflict with other libraries or your page defaults; it does not use global stylings.
Furthermore, you can also use Leonardo UI as a standalone with a CSS reset to get consistent styling of your components.
In the following example, you can see the standard styling of various components, such as buttons, input boxes, labels, functions, radio buttons, and switches:
This section will attempt to summarize the concept of Leonardo UI and give you the list of possibilities. There are, as always, many more examples and use cases to be found on the official website of...