How can users modify the appearance of a web map in ArcGIS Online?

Users can modify the appearance of a web map in ArcGIS Online primarily by adjusting layer properties and configuring the layout. This involves changing aspects such as the color schemes, symbols, and visibility of different layers, which enables users to create a map that effectively communicates the intended message or information.

Configuring the layout includes setting the layout of various map elements, such as titles, legends, and scale bars. This not only enhances the visual appeal but also improves the usability of the web map. By manipulating these properties, users can tailor their maps to meet specific audience needs or convey complex data more clearly.

The other options don't provide a comprehensive method for modifying the web map's appearance. Downloading a preset template may offer a starting point, but it doesn't allow for personal customization of existing elements. Importing external styles can be restrictive and may not apply well to all types of data present in the web map. Setting permissions for viewers focuses on who can see or interact with the map rather than how the map itself looks.