In order to view the formatted text formatted with a markup language, the "eye" must be clicked in the segmented button with the "pencil and eye"-symbols located in the toolbar.
Alternatively, the keyboard shortcut "CMD+<" can also be used to change the display, or you can switch between editor and preview via the main menu under the item "Display > Toggle Edit / Preview".
The display of the preview can be adjusted with CSS styles. More on this topic can be found on the page Changing the Preview Display with CSS Styles.